diff --git a/.agents/notes/implemented/bug-fix/2026-07-31-composer-glyph-layer-tracks-the-textarea.md b/.agents/notes/implemented/bug-fix/2026-07-31-composer-glyph-layer-tracks-the-textarea.md
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-# Agent Note: The composer's glyph layer tracks the textarea's scroll offset
-
-Status: implemented
-
-English | [中文](2026-07-31-composer-glyph-layer-tracks-the-textarea.zh.md)
-
-## Problem
-
-A composer draft longer than the 14-line cap could not be scrolled. The caret moved and the selection moved, but the words stayed frozen at line 1 — no wheel gesture, drag, or arrow key brought the end of a long draft on screen, so the bottom of anything past ~14 lines was unreachable and unreadable while writing it.
-
-The cap itself was working. The composer paints its text in two stacked layers ([InputBar](../../../../packages/client/ui-conversation/src/client/skeleton/InputBar.tsx)): the `<textarea>` owns the value, the selection, and the caret but renders its own glyphs `color: transparent`, and every visible character is painted by the `[data-input-backdrop]` div beneath it, which also carries the claim-token highlight, the chips, and the ghost hint. That split is what makes chips and highlights possible at all — a textarea cannot style a range of its own text.
-
-The two layers were coupled in geometry but not in scroll. The backdrop is `position: absolute; inset: 0; overflow: hidden`: it is clipped, not scrolled, and nothing in the browser links its offset to the textarea's. Below the cap that is invisible, because both layers rest at offset 0 and the mirror div sizes the box to the draft. At the cap the textarea starts scrolling and the backdrop does not follow, so the layer the user actually reads never moves.
-
-The defect is therefore exactly as old as the cap, and it hid behind the resting state: a short draft, the state every screenshot and every existing fixture captured, renders identically with and without the coupling.
-
-## Decision
-
-`InputBar` mirrors the textarea's `scrollTop` onto the backdrop from one `scroll` listener, registered beside the existing wheel-chaining listener in the same effect (the textarea is never unmounted — the inert state renders the same element disabled).
-
-One listener is the whole coupling, because every way the box moves ends in a `scroll` event on the textarea. A gesture scrolls it; an edit scrolls the caret into view; a draft that shrinks past the current offset clamps it. The clamp case is the one that looks like it needs separate handling and does not: the two layers share an extent, so they clamp to the same maximum, and the textarea's clamp fires the `scroll` that mirrors it.
-
-That shared extent is not free, and mirroring an offset is only correct while it holds. Two things break it, both discovered in review, both failing in the same direction — a backdrop shorter than the textarea, so the assignment clamps and the glyphs sit below the caret. A textarea reserves a line box for the caret after a final newline; `white-space: pre-wrap` collapses a text node's trailing newline and generates none. A draft ending in a newline therefore made the backdrop exactly one line shorter than the textarea — measured 628 against 652 — so the assignment clamped and the glyphs sat a line behind the caret at the very bottom. The backdrop now carries the same trailing-line sentinel the mirror div already did: its content is the decoration walk plus one `'\n'`, which the same collapse absorbs when the draft does not end in a newline and which supplies the missing line box when it does. Measured across plain, trailing-newline, soft-wrapping, unbreakable-run, and interior-blank-line drafts, the two extents now agree in every case.
-
-The second premise is wrap width, and it is asserted rather than fixed. Only `.input` scrolls, so only `.input` can lose content width to a scrollbar that consumes layout space, and a narrower `.input` wraps a long draft onto more lines — worth 2 to 5 lines for an 8px difference, measured on a standalone harness, while at equal widths a textarea and a div agree exactly. Measured on the running app across the three engines Playwright ships, the widths agree on two and not on the third:
-
-| engine | `.input` / `.backdrop` / `.mirror` wrap width | extents |
-|---|---|---|
-| chromium | 776 / 776 / 776 | equal |
-| firefox | 776 / 776 / 776 | equal |
-| WebKit | **768** / 776 / 776 | equal for the drafts measured |
-
-WebKit's textarea loses 8px to its scrollbar while the clipped layers keep theirs. That gap predates this change and is not closed here; the mirror is unaffected on the drafts measured because the extents still agree, but a draft whose wrapping is sensitive at exactly that width would make `.input` taller and clamp the mirrored offset. The scenario asserts the equality on the lane's engine, so a regression into that state fails loudly rather than silently.
-
-`scrollbar-gutter: stable` on the shared metrics block was tried and removed. WebKit applies it to `overflow-y: auto` but not to `overflow: hidden`, so it left `.input` at 768 against 776 — exactly the gap it was meant to close — while costing chromium 8px of text width unconditionally. Closing this needs one geometry every engine agrees on, not that property.
-
-The mirror is one-directional: the textarea is the authority because it owns the caret, and the caret is what the browser scrolls to.
-
-## Alternatives considered
-
-**Give the backdrop `overflow: auto` and let it scroll itself.** It would then have a scroll offset of its own to keep in step, which is the same problem plus a second scrollbar painted over the input. The backdrop is a projection of the textarea, not an independently navigable surface.
-
-**Drop the backdrop and style the textarea's own text.** This removes the layer split and the whole class of desync with it. Rejected because it is not implementable: a textarea renders one uniform text run, so the claim-token highlight, the chips, and the ghost hint — the reasons the backdrop exists — have no way to be expressed. Losing them to fix scrolling trades a bounded defect for a feature deletion.
-
-**Render the draft in a `contenteditable` div instead of a textarea.** One element, one scroll offset, styleable ranges. Rejected as far out of proportion to the defect: `contenteditable` would put IME composition, undo/redo, selection semantics, and paste normalization back on us, all of which the textarea plus the input machine currently handle, and the machine already owns an undo log that assumes a textarea's value semantics.
-
-**Scroll the backdrop from the existing wheel handler instead of a `scroll` listener.** The handler already runs on every wheel over the textarea, so it looks like the natural place. Rejected because it covers only one of the ways the box scrolls: typing at the end, `End`, arrow keys, drag-selection past the edge, and scrollbar drags all move the textarea without a wheel event. Listening to `scroll` is listening to the thing itself rather than to one of its causes.
-
-**Reserve the scrollbar gutter on all three layers with `scrollbar-gutter: stable`.** Adopted, then reverted on measurement. The reasoning was that whatever a platform's scrollbar costs, three layers reserving it stay equal — and `overflow: hidden` is a scroll container, so the spec says the clipped layers honour it. Chromium agrees (8px reserved on each, widths 768/768/768). WebKit does not: it reserves for `overflow-y: auto` and not for `overflow: hidden`, leaving 768 against 776 — the same gap, unclosed — so the property bought nothing on the one engine where the divergence is observable while costing every chromium user 8px of text column. Reverted in favour of asserting the premise and recording the WebKit gap.
-
-**Suppress the textarea's scrollbar instead of reserving a gutter on the other layers.** `scrollbar-width: none` on `.input` would equalize the widths without narrowing the text column. Rejected because the composer deliberately shows a thumb once the draft passes the cap — `.card` binds the l2 scrollbar tokens for exactly that — and removing it takes away the only affordance that says a long draft continues below.
-
-**Translate the backdrop with `transform: translateY(-scrollTop)` instead of scrolling it.** A transform is not clamped by content height, so it would paper over any extent divergence — including the trailing-newline one — without matching the layers. Rejected because the divergence is the actual defect: unequal extents also mean the two layers disagree about where the last line sits, and hiding that behind an unclamped transform would leave a mismatch that resurfaces the moment anything measures the backdrop. Fixing the extent keeps one truth about the draft's height.
-
-**Add a second mirror in a layout effect keyed on the committed draft.** This shipped in the first version of the change, on the theory that an edit reflows both layers without necessarily moving the textarea, and that a shrinking draft clamps each layer independently. Both premises are false, and it was removed after mutation-testing each hook alone against the built client: with only the layout effect disabled the browser scenario stays green, while disabling only the `scroll` listener fails it. Typing scrolls the caret into view, which is an ordinary `scroll`; a shrinking draft clamps both layers to the same maximum because their extents are equal, and the textarea's clamp fires `scroll` too. The specific hazard the effect was imagined to cover — React replacing the backdrop's children when the decoration set changes shape, resetting its offset — does not occur: measured in chromium, replacing every child of an `overflow: hidden` box preserves `scrollTop` (300 stays 300), and the only replacement that zeroes it is one that shrinks the content below the offset, which is the clamp case already covered.
-
-**Sync in the `onChange` handler.** Rejected for the same reason plus one of its own: it fires before React commits the new draft to the backdrop, so it would mirror against the previous layout.
-
-## Consequences
-
-- A draft past the cap scrolls its glyphs. Measured in the browser scenario: after a wheel gesture over a 40-line draft the last line sits inside the visible box and the first has scrolled out above it; before, the last line stayed a full draft-height below the box while the textarea's own offset had moved.
-- The coupling is one-directional and cheap — one assignment of one number, no measurement, no layout read beyond `scrollTop` — so it adds nothing to the typing path's cost.
-- Chips, claim-token highlights, and text-ref marks stay aligned with their glyphs while scrolled, because they are positioned inside the backdrop and move with it. Nothing about the decoration walk changes.
-- The composer's two-layer design keeps this hazard: any future layer added beside the backdrop needs the same mirroring, and any change to how a layer reserves its last line box breaks the extent equality the mirror depends on. The e2e scenario asserts both — the relation the user cares about (which line is on screen) and the extent equality underneath it — so a future divergence fails on the invariant rather than on a screenshot.
-- Extent equality is asserted, not assumed. It is the premise that turns "mirror the offset" from correct into subtly wrong, and it failed for the trailing-newline shape before the sentinel.
-- Wrap-width equality is the other premise, and it does NOT hold universally: WebKit lays `.input` out 8px narrower than the glyph layers. That predates this change and is left open, with the measurement recorded above and an assertion on the lane's engine. A draft whose wrapping turns on those 8px would clamp the mirror on WebKit.
-- The composer's layout is unchanged. An earlier revision narrowed the text column by 8px on every platform to chase the wrap-width premise; measurement showed it did not buy the guarantee, so the metrics are the same as before this change.
-
-## Testing
-
-The unit spec in [input-bar.spec.tsx](../../../../packages/client/ui-conversation/tests/input-bar.spec.tsx) proves the mirroring path runs: it stubs both offsets, because jsdom reports `scrollHeight === clientHeight` for every element and never scrolls one, and asserts the backdrop follows the textarea to a new offset and back to the top. Reverting the `ref` makes it fail.
-
-The user-visible fact needs a real engine, so [composer-draft-scroll.e2e.ts](../../../../apps/web/tests/composer-draft-scroll.e2e.ts) measures it in chromium against the built client: a 40-line draft in a fresh workspace's blank composer, zero model calls, with a DOM Range over the backdrop's own text reporting where the first and last lines sit relative to the visible box. A vacuity guard asserts the draft actually overflows the capped box first. A separate case drives the trailing-newline shape and asserts the two extents are equal before asserting the glyphs reach the end; each layer's maximum is observed by asking for an impossible offset and reading back the clamp, not computed from `scrollHeight`. A third asserts the gutter premise: equal wrap widths, and a reserved band greater than zero on each layer. The band is what keeps that assertion from being vacuous — the widths would also match with no reservation at all on this engine's overlay scrollbar, and it is the reservation, not the match, that carries the guarantee to a platform whose scrollbar takes real width.
-
-Confirmed both directions against the built client. With the mirroring reverted and the packages rebuilt, the wheel case fails on the layer offsets, the typing case fails with it, and the golden diff reads `last draft line is on screen: false` while `textarea moved: true` — the reported symptom stated as a fixture. The resting-state case passes in both builds, which is the point: it is the state that hid the defect.
-
-Note that the composer ships inside a client-module bundle, so `pnpm run build:web` alone does not pick up a change to `InputBar.tsx` — the package build must run for the browser lane to see it, and a scenario run against a stale `lib/` asserts against an older client than the tree.
diff --git a/.agents/notes/implemented/bug-fix/2026-07-31-composer-glyph-layer-tracks-the-textarea.zh.md b/.agents/notes/implemented/bug-fix/2026-07-31-composer-glyph-layer-tracks-the-textarea.zh.md
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-# Agent Note: composer 的字形层跟随 textarea 的滚动偏移
-
-Status: implemented
-
-[English](2026-07-31-composer-glyph-layer-tracks-the-textarea.md) | 中文
-
-## 问题
-
-草稿一旦超过 14 行的高度上限，就无法再滚动。光标会动，选区会动，但文字始终冻结在第 1 行——无论滚轮、拖拽还是方向键，都无法把长草稿的末尾带到可见范围内，因此约 14 行之后的内容在书写过程中既够不着也读不到。
-
-高度上限本身是正常工作的。composer 的文本由两层叠放绘制（见 [InputBar](../../../../packages/client/ui-conversation/src/client/skeleton/InputBar.tsx)）：`<textarea>` 持有取值、选区与光标，但它自己的字形以 `color: transparent` 渲染；用户看到的每一个字符都由其下的 `[data-input-backdrop]` 层绘制，该层同时承载 claim token 高亮、chip 与提示影子文本。这一拆分正是 chip 与高亮得以存在的前提——textarea 无法为自身文本的某个区间单独设置样式。
-
-两层在几何上是耦合的，在滚动上却不是。backdrop 为 `position: absolute; inset: 0; overflow: hidden`：它只做裁剪，不做滚动，浏览器也不会把它的偏移与 textarea 关联起来。未达上限时这一点不可见，因为两层都停在偏移 0，且镜像层会把盒子撑到草稿的高度。一旦触及上限，textarea 开始滚动而 backdrop 不跟随，于是用户真正在读的那一层从不移动。
-
-因此该缺陷与高度上限同龄，并且藏在静止状态背后：短草稿——也就是所有截图与既有 fixture（测试前置数据）所捕获的那个状态——在有无该耦合时渲染完全一致。
-
-## 决策
-
-`InputBar` 通过一个 `scroll` 监听把 textarea 的 `scrollTop` 镜像到 backdrop 上，该监听与既有的滚轮接力监听注册在同一个 effect 中（textarea 从不卸载——失效状态渲染的是同一个元素的 disabled 形态）。
-
-一个监听即构成完整耦合，因为这个盒子移动的每一种方式最终都会在 textarea 上产生 `scroll` 事件：手势使它滚动；编辑会把光标滚入可见范围；草稿缩短到当前偏移之下时它会被钳位。看似需要单独处理、实则不需要的正是钳位这一种：两层共享同一滚动范围，因此它们会钳位到同一个最大值，而 textarea 的钳位本身就会触发那次完成镜像的 `scroll`。
-
-这个「共享的滚动范围」并非白得，而镜像偏移只有在它成立时才是正确的。有两件事会破坏它，都是在审查中被发现的，且失效方向相同——backdrop 比 textarea 矮，于是赋值被钳制、字形落到光标之下。textarea 会在末尾换行之后为光标保留一个行盒，而 `white-space: pre-wrap` 会折叠文本节点的尾随换行、不生成任何行盒。因此以换行结尾的草稿会让 backdrop 恰好比 textarea 少一行——实测为 628 对 652——于是该赋值被钳制，滚到最底部时字形比光标落后一行。现在 backdrop 也带上了镜像层早已具备的同一枚尾行哨兵：其内容为装饰扫描的结果再加一个 `'\n'`；草稿不以换行结尾时它被同一次折叠吸收，以换行结尾时它补上缺失的那个行盒。对纯文本、尾随换行、软折行、不可断长串以及中间空行五类草稿实测，两侧范围在每种情形下均相等。
-
-第二个前提是折行宽度，它是被断言的，而不是被修复的。只有 `.input` 会滚动，因此也只有 `.input` 会把内容宽度让给一条占布局宽度的滚动条；`.input` 一旦更窄，长草稿就会折出更多行——在独立环境实测，8px 的宽度差值 2 到 5 行，而宽度相等时 textarea 与 div 完全一致。在运行中的应用上、对 Playwright 自带的三个引擎实测，两个相等、一个不等：
-
-| 引擎 | `.input` / `.backdrop` / `.mirror` 折行宽度 | 滚动范围 |
-|---|---|---|
-| chromium | 776 / 776 / 776 | 相等 |
-| firefox | 776 / 776 / 776 | 相等 |
-| WebKit | **768** / 776 / 776 | 所测草稿下相等 |
-
-WebKit 的 textarea 把 8px 让给了自己的滚动条，而两个被裁剪的图层没有。该差距先于本次改动存在，本 PR 未予关闭；在所测草稿下滚动范围仍然相等，因此镜像不受影响，但一份恰好在该宽度上折行敏感的草稿会让 `.input` 更高、从而钳制镜像偏移。场景在测试通道所用引擎上断言了这项相等性，因此一旦回退到那种状态会显式失败，而不是悄然发生。
-
-共享度量块上的 `scrollbar-gutter: stable` 曾被采用又被移除：WebKit 对 `overflow-y: auto` 应用它、对 `overflow: hidden` 不应用，于是 `.input` 仍是 768 对 776——正是它本想关闭的那个差距——同时又让 chromium 无条件损失 8px 文本宽度。要关闭它，需要一套所有引擎都认同的几何，而不是这个属性。
-
-该镜像是单向的：textarea 是权威方，因为它持有光标，而浏览器滚动的目标正是光标。
-
-## 曾考虑的替代方案
-
-**给 backdrop 加 `overflow: auto`，让它自行滚动。** 那样它就有了一个属于自己的滚动偏移需要同步，问题原样保留，还额外多出一条画在输入框上的滚动条。backdrop 是 textarea 的投影，而不是一个可独立导航的界面。
-
-**去掉 backdrop，直接为 textarea 自身文本设置样式。** 这会消除分层，连同整类失步问题一并消除。之所以否决，是因为它根本无法实现：textarea 只渲染一段统一的文本流，因此 claim token 高亮、chip 与提示影子文本——backdrop 存在的全部理由——都无从表达。为修滚动而放弃它们，是拿一个有界的缺陷去换一次功能删除。
-
-**改用 `contenteditable` div 承载草稿，不再用 textarea。** 一个元素、一个滚动偏移、区间可设样式。之所以否决，是它与该缺陷的体量严重不相称：`contenteditable` 会把 IME 组词、撤销／重做、选区语义与粘贴规范化重新压回我们身上，而这些目前都由 textarea 加输入状态机处理，且状态机已持有一份以 textarea 取值语义为前提的撤销日志。
-
-**在既有的滚轮处理函数里滚动 backdrop，而不是新增 `scroll` 监听。** 该处理函数本就在 textarea 上的每次滚轮时运行，看似是自然的落点。之所以否决，是它只覆盖了盒子滚动的其中一种成因：在末尾输入、`End`、方向键、拖选越过边缘、拖动滚动条，都会在没有滚轮事件的情况下移动 textarea。监听 `scroll` 是在监听事情本身，而不是它的某一个成因。
-
-**用 `scrollbar-gutter: stable` 让三层一起预留滚动条 gutter。** 曾经采用，实测后回退。当初的推理是：无论平台滚动条占多少宽度，三层都预留同样多即可保持相等；而且 `overflow: hidden` 也是滚动容器，按规范应当遵守该声明。chromium 确实如此（三层各预留 8px，宽度 768/768/768）。WebKit 不然：它对 `overflow-y: auto` 预留、对 `overflow: hidden` 不预留，结果仍是 768 对 776——差距原样保留——于是该属性在唯一能观测到这一偏差的引擎上一无所获，却让每一位 chromium 用户损失 8px 文本列。改为断言该前提并记录 WebKit 的差距。
-
-**改为抑制 textarea 的滚动条，而不是给另外两层预留 gutter。** 在 `.input` 上写 `scrollbar-width: none` 同样能让宽度相等，且不必收窄文本列。之所以否决：草稿超过上限后 composer 是有意显示滚动条滑块的——`.card` 正是为此绑定了 l2 滚动条 token——去掉它就等于拿走了「下面还有内容」这一唯一提示。
-
-**改用 `transform: translateY(-scrollTop)` 平移 backdrop，而不是滚动它。** transform 不受内容高度钳制，因此它能把任何范围偏差——包括尾随换行这一种——一并掩盖，却并不让两层真正对齐。之所以否决，是因为这个偏差本身就是真正的缺陷：范围不等同时意味着两层对末行位置的判断不一致，把它藏在一个不受钳制的 transform 之后，只会让这一失配在任何人去测量 backdrop 的那一刻重新浮现。修正范围本身，才能让草稿高度只有一个事实来源。
-
-**再加一个以已提交草稿为 key 的 layout effect 作为第二道镜像。** 该改动的第一版确实带着它，理由是：一次编辑会让两层重排却不一定让 textarea 移动，且草稿变短时两层各自独立地被钳位。这两个前提都不成立，因此在针对构建产物客户端逐个变异测试每个 hook 之后将其移除：仅禁用 layout effect 时浏览器场景全绿，而仅禁用 `scroll` 监听则会失败。输入会把光标滚入可见范围，那就是一次普通的 `scroll`；草稿变短时两层因范围相等而钳位到同一个最大值，且 textarea 的钳位同样会触发 `scroll`。该 effect 本想覆盖的那个具体隐患——React 在装饰集合形状变化时替换 backdrop 的全部子节点，从而重置其偏移——并不会发生：在 chromium 中实测，替换一个 `overflow: hidden` 盒子的全部子节点会保留 `scrollTop`（300 仍为 300），唯一会将其归零的替换是把内容缩短到偏移之下，而那正是已被覆盖的钳位情形。
-
-**在 `onChange` 处理函数里同步。** 除上述同样的理由外还有其自身的问题：它在 React 把新草稿提交到 backdrop 之前触发，因而会按上一次的布局做镜像。
-
-## 后果
-
-- 超过上限的草稿会滚动其字形。浏览器场景实测：在 40 行草稿上做一次滚轮手势后，最后一行位于可见盒子之内，第一行已滚出上方；此前最后一行仍停在盒子下方整整一个草稿高度处，而 textarea 自身的偏移已经移动了。
-- 该耦合是单向且廉价的——一次对一个数字的赋值，没有测量，除 `scrollTop` 外没有额外的布局读取——因此不会给输入路径增加开销。
-- chip、claim token 高亮与文本引用标记在滚动时始终与其字形对齐，因为它们定位在 backdrop 内部并随之移动。装饰扫描本身没有任何改动。
-- composer 的双层设计保留了这一隐患：日后在 backdrop 旁新增的任何一层都需要同样的镜像；而任何改变某一层如何保留其末行行盒的改动，都会破坏镜像所依赖的范围相等性。e2e 场景对两者都做了断言——用户真正关心的关系（哪一行在屏幕上），以及其下的范围相等性——因此日后一旦出现偏差，失败会落在不变量上，而不是落在某张截图上。
-- 范围相等性是被断言的，而非被假定的。它正是那个能把「镜像偏移」从正确变为微妙错误的前提，并且在加入哨兵之前，它在尾随换行这一形态上确实不成立。
-- 折行宽度相等是另一个前提，而它并非普遍成立：WebKit 把 `.input` 排得比字形层窄 8px。该问题先于本次改动存在，此处保持开放，上文记录了实测数值，并在测试通道所用引擎上加了断言。一份折行恰好取决于这 8px 的草稿会在 WebKit 上钳制镜像。
-- composer 的布局没有变化。此前有一版为追求折行宽度前提而在所有平台把文本列收窄了 8px；实测表明它并不能带来该保证，因此度量与改动前保持一致。
-
-## 验证
-
-[input-bar.spec.tsx](../../../../packages/client/ui-conversation/tests/input-bar.spec.tsx) 中的单元用例证明镜像路径确实执行：它对两侧偏移都做了桩替换——因为 jsdom 对任何元素都报告 `scrollHeight === clientHeight` 且从不滚动任何元素——并断言 backdrop 既跟随 textarea 到新的偏移，也跟随它回到顶部。撤掉那个 `ref` 会让它失败。
-
-用户可见的事实需要真实引擎，因此 [composer-draft-scroll.e2e.ts](../../../../apps/web/tests/composer-draft-scroll.e2e.ts) 在 chromium 中针对构建产物客户端测量它：在全新工作区空白会话的 composer 中放入 40 行草稿，零模型调用，用一个跨越 backdrop 自身文本的 DOM Range 报告首行与末行相对于可见盒子的位置。一个防空转守卫会先断言草稿确实溢出了设有上限的盒子。另有一个独立用例驱动尾随换行这一形态，先断言两侧范围相等，再断言字形确实抵达末尾；每一层的最大值都通过请求一个不可能的偏移再读回其钳位结果来观测，而非由 `scrollHeight` 计算得出。第三个用例断言 gutter 前提：折行宽度相等，且每层预留的带宽大于零。正是这条「带宽」使该断言不至于空转——在本引擎的 overlay 滚动条下，即使完全不预留，两侧宽度也会相等；把保证传递到滚动条真正占宽的平台上的，是那次预留，而不是这次相等。
-
-已双向确认。撤掉镜像并重新构建各包后，滚轮用例在两层偏移上失败，输入用例随之失败，golden 差异读作 `last draft line is on screen: false` 而 `textarea moved: true`——即以 fixture（测试前置数据）形式陈述的原始现象。静止状态用例在两种构建下都通过，这正是要点所在：它就是掩盖了该缺陷的那个状态。
-
-注意 composer 随客户端模块 bundle 一同发布，因此仅运行 `pnpm run build:web` 不会纳入对 `InputBar.tsx` 的改动——必须运行包构建，浏览器测试通道才能看到它；针对陈旧 `lib/` 运行的场景，断言的是比当前工作树更旧的客户端。
diff --git a/.agents/notes/implemented/bug-fix/2026-07-31-composer-glyph-layer-tracks-the-textarea.i18n.yaml b/.agents/notes/implemented/bug-fix/2026-07-31-composer-text-layers-share-one-scrollport.i18n.yaml
similarity index 53%
rename from .agents/notes/implemented/bug-fix/2026-07-31-composer-glyph-layer-tracks-the-textarea.i18n.yaml
rename to .agents/notes/implemented/bug-fix/2026-07-31-composer-text-layers-share-one-scrollport.i18n.yaml
index c95b2a60b7..56f5dc114f 100644
--- a/.agents/notes/implemented/bug-fix/2026-07-31-composer-glyph-layer-tracks-the-textarea.i18n.yaml
+++ b/.agents/notes/implemented/bug-fix/2026-07-31-composer-text-layers-share-one-scrollport.i18n.yaml
@@ -1,6 +1,6 @@
 # Bilingual-pair consistency record (docs/i18n/README.md): the git blob hash of each
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
-#   pnpm run verify-translation-pairing --write .agents/notes/implemented/bug-fix/2026-07-31-composer-glyph-layer-tracks-the-textarea.md
-2026-07-31-composer-glyph-layer-tracks-the-textarea.md: d60a100be98683b5f7a7c88edf7585d275134730
-2026-07-31-composer-glyph-layer-tracks-the-textarea.zh.md: eab3f9e3fe3bddb426836113d08f1839329119d5
+#   pnpm run verify-translation-pairing --write .agents/notes/implemented/bug-fix/2026-07-31-composer-text-layers-share-one-scrollport.md
+2026-07-31-composer-text-layers-share-one-scrollport.md: ef182f6a62dabfaf5330916c45baf98d1543e6e5
+2026-07-31-composer-text-layers-share-one-scrollport.zh.md: b6f5534300f302ff58022733c3493af5eb53428f
diff --git a/.agents/notes/implemented/bug-fix/2026-07-31-composer-text-layers-share-one-scrollport.md b/.agents/notes/implemented/bug-fix/2026-07-31-composer-text-layers-share-one-scrollport.md
new file mode 100644
index 0000000000..ef182f6a62
--- /dev/null
+++ b/.agents/notes/implemented/bug-fix/2026-07-31-composer-text-layers-share-one-scrollport.md
@@ -0,0 +1,73 @@
+# Agent Note: The composer's two text layers share one scrollport
+
+Status: implemented
+
+English | [中文](2026-07-31-composer-text-layers-share-one-scrollport.zh.md)
+
+## Problem
+
+The composer paints its text in two stacked layers ([InputBar](../../../../packages/client/ui-conversation/src/client/skeleton/InputBar.tsx)): the `<textarea>` owns the value, the selection, and the caret but renders its own glyphs `color: transparent`, and every visible character is painted by the `[data-input-backdrop]` div beneath it, which also carries the claim-token highlight, the chips, and the ghost hint. That split is what makes chips and highlights possible at all — a textarea cannot style a range of its own text. The draft box is capped at 14 lines, so past the cap something has to scroll.
+
+Two layers with two scroll offsets fail in two stages, and this change is the second one.
+
+The first was static. The backdrop is `position: absolute; inset: 0; overflow: hidden` — clipped, not scrolled — and nothing in the browser links its offset to the textarea's, so past the cap the caret moved and the words stayed frozen at line 1. Below the cap that is invisible, because both layers rest at 0; the defect was exactly as old as the cap and hid behind the resting state that every screenshot captured. It was fixed by mirroring the textarea's `scrollTop` onto the backdrop from a `scroll` listener, and that made the two layers agree **at rest**.
+
+The second is what a user then reported: swiping quickly from the top of a long draft throws the caret up out of its own text, as though it carried momentum, and it settles back a moment later. The mirror is the cause. A wheel gesture scrolls the textarea on the compositor, off the main thread; the `scroll` event that drives the assignment is dispatched afterwards, so for the frames in between the caret sits at the new offset and every glyph sits at the old one. Measured on a standalone harness of the same geometry, moving the offset 200px and reading the caret's distance to its own glyphs before the task ends: chromium 203px, firefox 202px, WebKit 203px of separation, settling to the fixed line-box constant (3/2/3) a frame or two later. Every engine, every gesture, proportional to how fast the user scrolls.
+
+No listener can close that gap, because the gap is the definition of a listener: it runs after the thing it reacts to. Anything that keeps two boxes equal in JavaScript is a frame behind a compositor that moves one of them without asking.
+
+## Decision
+
+One scrolling box, holding both layers.
+
+`[data-input-scroll]` is the composer's only scrollport and carries the 14-line cap. Inside it, the auto-grow stack is as tall as the **whole** draft — the hidden mirror div is in normal flow and no longer capped, so it sizes the stack to the full text — and the backdrop and textarea ride that height absolutely. The textarea is `overflow: hidden` with no scrollable overflow of its own; it can no longer hold an offset at all.
+
+The browser then applies one offset to both layers, in the same frame, on the same compositor. The caret is bound to its glyphs by construction rather than by upkeep: there is no code to run, no event to wait for, and no state that can be one frame stale. The wheel-chaining handler stays, retargeted from the textarea to the scrollport, and remains the only listener on the box.
+
+Two things the previous mechanism needed are gone with it:
+
+**The backdrop's trailing-line sentinel.** It existed to keep the two boxes' scroll extents equal — a textarea reserves a line box for the caret after a final newline while `white-space: pre-wrap` collapses a text node's trailing newline, so a draft ending in a newline made the backdrop one line shorter and clamped the mirrored offset a line above the caret. With one scrollport the backdrop's own extent decides nothing: the mirror div sizes the stack for both layers, both start at the same top, and a layer whose content ends earlier simply paints nothing on the last line.
+
+**The wrap-width premise.** All three layers now resolve their width inside the scrollport, so a scrollbar that consumes layout space costs them the same width by construction. This closes the divergence the superseded note recorded as open and unfixable by any property: WebKit reserved gutter space for the `overflow-y: auto` textarea and not for the `overflow: hidden` layers beside it, laying the textarea out 768 against 776 — worth 2 to 5 wrapped lines on a long draft, i.e. glyphs under the wrong caret on the one engine where it was observable. Measured on the harness after the change, all three layers report one width on all three engines Playwright ships.
+
+Revealing the caret is the one thing that now depends on the browser rather than on us: with no offset of its own, the textarea's scroll-into-view has to walk up to the scrollport. It does, on every engine measured — typing at the draft's end brings the scrollport to the caret (625, 626 and 628 of a 628px maximum in chromium, firefox and WebKit), walking the caret back up with `ArrowUp` scrolls back to it, and typing after scrolling away returns to it.
+
+## Alternatives considered
+
+**Mirror `scrollTop` onto the backdrop from a `scroll` listener.** The superseded decision, and correct at rest: it is what made a long draft scrollable at all. Rejected now because it cannot be correct in motion — it is a main-thread reaction to a compositor-thread fact — and because it needed two premises to stay true that the single scrollport does not need at all (equal extents, equal wrap widths), each of which had already failed once.
+
+**Translate the backdrop with `transform: translateY(-scrollTop)` instead of assigning an offset.** Same lag: still main-thread, still driven by the same event. It additionally papers over extent divergence rather than making the layers agree, so a mismatch resurfaces the moment anything measures the backdrop.
+
+**Drive the backdrop from a scroll-driven animation (`animation-timeline: scroll()`).** This would run the coupling on the compositor and genuinely eliminate the lag while keeping two boxes. Rejected on support: Safari does not implement it and Firefox has only recently, so the composer would keep the reported defect on the engines that lack it, and the fallback path would be the mechanism being replaced.
+
+**Scroll both layers from JavaScript, with the textarea `overflow: hidden` and a wheel handler assigning both offsets in one task.** No divergence during wheel gestures, since nothing scrolls without us. Rejected because it replaces native scrolling — momentum, trackpad rubber-banding, scrollbar dragging, keyboard scrolling — with a hand-written approximation, and the caret-reveal path (the browser setting the textarea's own offset) still lands asynchronously.
+
+**Keep the cap on the mirror and just wrap today's structure in a scroller.** The layers would stay window-sized, not draft-sized: `inset: 0` on an absolutely positioned child resolves against the scrollport's padding box, not its scrollable overflow area, so both layers would scroll away from the content that is supposed to be underneath them. The stack has to be the full draft height for the arrangement to mean anything.
+
+**Give the backdrop `overflow: auto` and let it scroll itself.** It would then have an offset of its own to keep in step, which is the same problem plus a second scrollbar painted over the input. The backdrop is a projection of the textarea, not an independently navigable surface.
+
+**Drop the backdrop and style the textarea's own text.** This removes the layer split and the whole class of desync with it. Rejected because it is not implementable: a textarea renders one uniform text run, so the claim-token highlight, the chips, and the ghost hint — the reasons the backdrop exists — have no way to be expressed. Losing them to fix scrolling trades a bounded defect for a feature deletion.
+
+**Render the draft in a `contenteditable` div.** One element, one offset, styleable ranges. Rejected as far out of proportion: `contenteditable` would put IME composition, undo/redo, selection semantics, and paste normalization back on us, all of which the textarea plus the input machine currently handle, and the machine already owns an undo log that assumes a textarea's value semantics.
+
+**`scrollbar-gutter: stable` on all three layers.** Tried and reverted while the textarea was the scroller: WebKit applied it to `overflow-y: auto` and not to `overflow: hidden`, leaving the same 8px gap unclosed while costing every chromium user 8px of text column. Moot now — the layers share a containing block, so there is nothing to reserve.
+
+**`scrollbar-width: none` on the scrolling layer.** Would equalize widths by hiding the thumb. Rejected: the composer deliberately shows one once the draft passes the cap — `.card` binds the l2 scrollbar tokens for exactly that — and it is the only affordance saying a long draft continues below.
+
+## Consequences
+
+- The caret cannot leave its glyphs. The browser scrolls one box, so the separation between where the textarea puts a line and where the backdrop paints it is a fixed line-box constant at every offset, mid-gesture included. The scenario measures exactly that number.
+- The scrollbar moved from the textarea to the scrollport — the same visual place, one box out. The `.card` l2 token binding still inherits down to it.
+- Chips, claim-token highlights, and text-ref marks stay aligned with their glyphs while scrolled, because they are positioned inside the backdrop and move with it. The decoration walk is unchanged apart from the dropped sentinel.
+- On Firefox and WebKit, clicking into a composer whose draft overflows the cap now also scrolls the conversation transcript to its bottom: the caret's scroll-into-view walks past the composer's scrollport up to the transcript scrollport, which a textarea shorter than its box never made it do. Chromium does not. Measured, with `overscroll-behavior: contain` and `contain: paint` both tried and neither stopping the walk — there is no CSS that ends scroll-into-view chaining. Accepted: it scrolls toward the bottom, where the composer already sits, and the alternative is a caret visibly detached from its text on every engine.
+- Any layer added beside the backdrop belongs INSIDE the scrollport and must be as tall as the draft, or it reintroduces exactly this defect. This is the composer's standing hazard: the two-layer split is load-bearing for chips and highlights, so the coupling has to be structural, not maintained.
+
+## Testing
+
+The unit spec in [input-bar.spec.tsx](../../../../packages/client/ui-conversation/tests/input-bar.spec.tsx) asserts what jsdom can see: that one scrolling box contains both the textarea and the backdrop, and that the backdrop's text is now the draft and nothing else. jsdom reports `scrollHeight === clientHeight` for every element and never scrolls one, so the geometry belongs to the browser scenario; the wheel-chaining cases stub the scrollport's metrics rather than the textarea's.
+
+[composer-draft-scroll.e2e.ts](../../../../apps/web/tests/composer-draft-scroll.e2e.ts) measures the rest in chromium against the built client: a 40-line draft in a fresh workspace's blank composer, zero model calls. Every metric is read in the caret's own coordinate frame — where the textarea places line n, offset included — against a DOM Range over the backdrop's text for the same line, because that difference is what a user sees. The decisive case changes the offset and re-reads that difference **before the task ends**, which is before any `scroll` listener could have run: 0 with one scrollport, and the full delta with a mirror. A vacuity guard asserts the draft overflows the capped box first, and separate cases cover the cap, one wrap width across all three layers, a wheel gesture, a trailing-newline draft, and the caret-reveal path that the textarea's own scrolling used to handle — typing after scrolling away must bring the scrollport back to the caret.
+
+The two-geometry comparison behind the decision was measured on a standalone harness before implementing, since the old and new arrangements cannot both exist in the app at once: the same-task separation is 203/202/203px old against 3/2/3px new (chromium/firefox/WebKit), the wrap widths 768-against-776 old on WebKit against 1264/1264/1264 new on all three, and the textarea's own scrollable overflow 0 in the new geometry, which is what makes a second offset impossible rather than merely equal.
+
+Note that the composer ships inside a client-module bundle, so `pnpm run build:web` alone does not pick up a change to `InputBar.tsx` — the package build must run for the browser lane to see it, and a scenario run against a stale `lib/` asserts against an older client than the tree.
diff --git a/.agents/notes/implemented/bug-fix/2026-07-31-composer-text-layers-share-one-scrollport.zh.md b/.agents/notes/implemented/bug-fix/2026-07-31-composer-text-layers-share-one-scrollport.zh.md
new file mode 100644
index 0000000000..b6f5534300
--- /dev/null
+++ b/.agents/notes/implemented/bug-fix/2026-07-31-composer-text-layers-share-one-scrollport.zh.md
@@ -0,0 +1,73 @@
+# Agent Note: composer 的两层文本共用同一个滚动容器
+
+Status: implemented
+
+[English](2026-07-31-composer-text-layers-share-one-scrollport.md) | 中文
+
+## 问题
+
+composer 的文本由两层叠放绘制（见 [InputBar](../../../../packages/client/ui-conversation/src/client/skeleton/InputBar.tsx)）：`<textarea>` 持有取值、选区与光标，但它自己的字形以 `color: transparent` 渲染；用户看到的每一个字符都由其下的 `[data-input-backdrop]` 层绘制，该层同时承载 claim token 高亮、chip 与提示影子文本。这一拆分正是 chip 与高亮得以存在的前提——textarea 无法为自身文本的某个区间单独设置样式。草稿框的高度上限为 14 行，因此超过上限之后总得有东西滚动。
+
+两层各自持有一个滚动偏移，会分两个阶段失效，本次改动是其中的第二个。
+
+第一个阶段是静态的。backdrop 为 `position: absolute; inset: 0; overflow: hidden`——只裁剪、不滚动——浏览器也不会把它的偏移与 textarea 关联起来，于是超过上限之后光标在动而文字冻结在第 1 行。未达上限时这一点不可见，因为两层都停在 0；该缺陷与高度上限同龄，藏在所有截图所捕获的那个静止状态背后。当时的修复是用一个 `scroll` 监听把 textarea 的 `scrollTop` 镜像到 backdrop 上，这让两层在**静止时**保持一致。
+
+第二个阶段正是随后被用户报告的现象：从长草稿的顶部快速滑到底部时，光标像带着惯性一样从自己的文字里往上飞出去，过一会儿又落回原位。原因就是那个镜像。滚轮手势在合成线程上滚动 textarea，不经过主线程；而驱动赋值的 `scroll` 事件在其后才派发，因此中间这些帧里光标位于新偏移、每一个字形却仍位于旧偏移。在同一套几何的独立环境上实测：把偏移改变 200px 并在本任务结束之前读取光标与其字形之间的距离，chromium 分离 203px、firefox 202px、WebKit 203px，一两帧之后才收敛到固定的行盒常量（3/2/3）。每个引擎、每次手势都如此，且滑动越快分离越远。
+
+任何监听都无法消除这个间隙，因为这个间隙正是监听的定义：它在自己所响应的那件事之后才运行。凡是用 JavaScript 维持两个盒子相等的做法，都会落后于那个不经询问就搬动其中一个盒子的合成器一帧。
+
+## 决策
+
+只保留一个滚动盒，让它同时装下两层。
+
+`[data-input-scroll]` 是 composer 唯一的滚动容器，14 行的高度上限落在它身上。容器内部的自增高栈与**整份**草稿等高——隐藏的镜像层处于常规流中且不再设上限，因此由它把栈撑到完整文本高度——backdrop 与 textarea 以绝对定位骑在这个高度上。textarea 为 `overflow: hidden`，自身没有可滚动溢出，也就再无法持有任何偏移。
+
+于是浏览器在同一帧、同一个合成器上，把同一个偏移施加给两层。光标与字形的绑定来自结构本身，而不是来自持续维护：没有代码要跑，没有事件要等，也没有任何状态可能落后一帧。滚轮接力处理器保留，只是从 textarea 改挂到滚动容器上，并且仍是这个盒子上唯一的监听。
+
+上一版机制所需要的两样东西随它一起消失：
+
+**backdrop 的尾行哨兵。** 它的存在只是为了让两个盒子的滚动范围相等——textarea 会在末尾换行之后为光标保留一个行盒，而 `white-space: pre-wrap` 会折叠文本节点的尾随换行，因此以换行结尾的草稿会让 backdrop 少一行，把镜像偏移钳制在光标上方一行。改为单一滚动容器后，backdrop 自身的范围不再决定任何事：镜像层为两层统一定高，两层顶端对齐，内容更早结束的那一层只是在最后一行什么都不画。
+
+**折行宽度这一前提。** 现在三层都在滚动容器内部解析自身宽度，因此一条占布局宽度的滚动条对它们的代价由结构保证相等。这也就关闭了被取代的那篇笔记记录为「悬置且没有任何属性能修」的分歧：WebKit 会为 `overflow-y: auto` 的 textarea 预留槽位，却不为它旁边 `overflow: hidden` 的层预留，把 textarea 排成 768 对 776——在长草稿上值 2 到 5 个折行，也就是在唯一能观察到它的那个引擎上把字形放到了错误的光标之下。改动后在独立环境实测，Playwright 自带的三个引擎上三层宽度均一致。
+
+现在唯一依赖浏览器而非依赖我们自己的，是把光标滚入可见范围：textarea 没有了自己的偏移，它的 scroll-into-view 必须向上走到滚动容器。实测的每个引擎都会这么做——在草稿末尾输入会把滚动容器带到光标处（chromium、firefox、WebKit 分别为 625、626、628，最大值 628），用 `ArrowUp` 把光标一路走回去会滚回去，滚离光标后再输入也会回到光标。
+
+## 备选方案
+
+**用 `scroll` 监听把 `scrollTop` 镜像到 backdrop 上。** 被取代的那个决策，静止时是正确的：正是它让长草稿第一次可以滚动。现在被否决，是因为它在运动中不可能正确——它是主线程对合成线程事实的响应——并且它需要两个前提持续成立（范围相等、折行宽度相等），而单一滚动容器根本不需要这两个前提，其中每一个都已经失效过一次。
+
+**用 `transform: translateY(-scrollTop)` 平移 backdrop，而不是赋一个偏移。** 延迟相同：仍在主线程，仍由同一个事件驱动。它还会把范围分歧盖住而不是让两层真正一致，于是只要有什么东西去测量 backdrop，错配就会重新浮现。
+
+**用滚动驱动动画（`animation-timeline: scroll()`）驱动 backdrop。** 这会把耦合放到合成器上运行，在保留两个盒子的前提下确实能消除延迟。因支持度被否决：Safari 尚未实现、Firefox 也是近期才有，因此在缺少它的引擎上 composer 会保留被报告的缺陷，而其回退路径正是这次要替换掉的机制。
+
+**两层都由 JavaScript 驱动滚动：textarea 设 `overflow: hidden`，滚轮处理器在同一个任务里给两个偏移赋值。** 滚轮手势期间不会分离，因为没有我们就没有东西会滚动。被否决，是因为它用手写近似替换了原生滚动——惯性、触控板回弹、拖拽滚动条、键盘滚动——而且光标回视路径（浏览器设置 textarea 自己的偏移）仍然是异步落地的。
+
+**把上限留在镜像层上，只在今天的结构外面套一个滚动容器。** 那样两层仍是「窗口大小」而非「草稿大小」：绝对定位子元素的 `inset: 0` 是相对滚动容器的 padding box 解析的，而不是相对其可滚动溢出区域，于是两层会从本该垫在它们下面的内容上滚开。栈必须与整份草稿等高，这套排布才有意义。
+
+**给 backdrop 加 `overflow: auto`，让它自己滚动。** 那样它就有了一个自己的偏移需要保持同步，即同一个问题再加一条画在输入框上的滚动条。backdrop 是 textarea 的投影，不是一个可独立导航的界面。
+
+**去掉 backdrop，直接给 textarea 自己的文本上样式。** 这会消除分层，也一并消除这一整类失步。被否决是因为它不可实现：textarea 只渲染一段统一的文本，claim token 高亮、chip 与提示影子文本——backdrop 存在的理由——无从表达。为修滚动而失去它们，是拿一个有界缺陷换一次功能删除。
+
+**用 `contenteditable` div 渲染草稿。** 一个元素、一个偏移、区间可上样式。因代价与缺陷严重不成比例被否决：`contenteditable` 会把输入法组合、撤销/重做、选区语义与粘贴规范化重新压回我们身上，而这些目前都由 textarea 加输入状态机处理，且状态机已经持有一份假定 textarea 取值语义的撤销日志。
+
+**给三层都加 `scrollbar-gutter: stable`。** 在 textarea 还是滚动者时试过并已回退：WebKit 只对 `overflow-y: auto` 生效、不对 `overflow: hidden` 生效，那 8px 的差距原样留着，却让每个 chromium 用户无条件损失 8px 文本列宽。现在已无意义——三层共享同一包含块，没有什么需要预留。
+
+**给滚动的那一层加 `scrollbar-width: none`。** 靠隐藏滑块来抹平宽度。被否决：草稿超过上限时 composer 是有意显示滑块的——`.card` 绑定 l2 滚动条 token 正是为此——而它是唯一提示「长草稿在下面还有」的可供性。
+
+## 影响
+
+- 光标不可能离开自己的字形。浏览器滚动的是同一个盒子，因此「textarea 把某一行放在哪」与「backdrop 把这一行画在哪」之间的距离在任何偏移下都是一个固定的行盒常量，手势进行中也不例外。场景测试度量的正是这个数。
+- 滚动条从 textarea 移到了滚动容器上——视觉位置相同，只是外移了一层。`.card` 的 l2 token 绑定仍会继承下去。
+- chip、claim token 高亮与文本引用标记在滚动时仍与其字形对齐，因为它们定位在 backdrop 内部、随之移动。除去掉哨兵之外，装饰扫描没有变化。
+- 在 Firefox 与 WebKit 上，点进一个草稿超过上限的 composer 现在还会把会话记录滚动到底部：光标的 scroll-into-view 会越过 composer 的滚动容器一路走到会话记录的滚动容器，而一个比自身盒子矮的 textarea 从不会引发这一步。chromium 不会。已实测，并试过 `overscroll-behavior: contain` 与 `contain: paint`，两者都拦不住这次上行——没有任何 CSS 能终止 scroll-into-view 的接力。接受：它滚向底部，而 composer 本来就在底部，而其替代方案是在每个引擎上都出现光标与文字明显分离。
+- 任何新增在 backdrop 旁边的层都属于滚动容器**内部**，并且必须与草稿等高，否则就会重新引入这一缺陷。这是 composer 长期存在的风险点：两层拆分对 chip 与高亮是承重的，因此耦合必须来自结构，而不是靠维护。
+
+## 测试
+
+[input-bar.spec.tsx](../../../../packages/client/ui-conversation/tests/input-bar.spec.tsx) 中的单元用例断言 jsdom 能看见的部分：同一个滚动盒同时包含 textarea 与 backdrop，且 backdrop 的文本现在就是草稿本身、不多不少。jsdom 对任何元素都报告 `scrollHeight === clientHeight` 且从不滚动，因此几何属于浏览器场景；滚轮接力用例改为桩接滚动容器的度量，而非 textarea 的。
+
+[composer-draft-scroll.e2e.ts](../../../../apps/web/tests/composer-draft-scroll.e2e.ts) 在 chromium 中针对构建产物度量其余部分：全新工作区空会话的 composer 中一份 40 行草稿，零模型调用。每个度量都在光标自己的坐标系里读取——即 textarea 把第 n 行放在哪，含其自身偏移——再与 backdrop 同一行文本上的 DOM Range 相比，因为这个差值正是用户看到的东西。决定性的用例改变偏移，并**在本任务结束之前**重新读取该差值，也就是在任何 `scroll` 监听可能运行之前：单一滚动容器下为 0，镜像方案下则是整个增量。空洞性保护先断言草稿确实超过了带上限的盒子；其余用例分别覆盖高度上限、三层同一折行宽度、滚轮手势、以换行结尾的草稿，以及过去由 textarea 自身滚动承担的光标回视路径——滚离光标后输入，必须把滚动容器带回光标处。
+
+支撑该决策的两套几何对比是在实现之前于独立环境度量的，因为新旧排布无法在应用里同时存在：同任务分离度旧为 203/202/203px、新为 3/2/3px（chromium/firefox/WebKit），折行宽度旧在 WebKit 上为 768 对 776、新在三个引擎上均为 1264/1264/1264，而新几何下 textarea 自身的可滚动溢出为 0——正是这一点让第二个偏移不可能存在，而不只是碰巧相等。
+
+注意 composer 打包在 client-module bundle 内，因此只跑 `pnpm run build:web` 并不会带上 `InputBar.tsx` 的改动——必须先跑包构建，浏览器泳道才看得到；对着过期 `lib/` 跑场景，断言的是比当前代码树更旧的客户端。
diff --git a/apps/web/tests/approval-composer.e2e.ts b/apps/web/tests/approval-composer.e2e.ts
index b66277e0b2..d66cfc8f0f 100644
--- a/apps/web/tests/approval-composer.e2e.ts
+++ b/apps/web/tests/approval-composer.e2e.ts
@@ -77,12 +77,14 @@ describe('web e2e: approval takeover keeps its actions reachable', () => {
     const input = page.locator('textarea').first()
     await input.waitFor({ timeout: 10_000 })
 
-    // The composer's own text cap, measured on the live textarea before the
-    // takeover replaces it. The panel's scroll region must stop at the same
-    // height (the designer's requirement: one cap for the composer seat), and
-    // measuring it here keeps the assertion free of the px value itself.
+    // The composer's own text cap, measured on the live draft scrollport before
+    // the takeover replaces it — the box that carries the cap, while the
+    // textarea inside it is as tall as the whole draft. The panel's scroll
+    // region must stop at the same height (the designer's requirement: one cap
+    // for the composer seat), and measuring it here keeps the assertion free of
+    // the px value itself.
     await input.fill(CAP_PROBE)
-    const composerCap = await input.evaluate(el => el.clientHeight)
+    const composerCap = await input.evaluate(el => el.closest('[data-input-scroll]')?.clientHeight ?? 0)
     expect(composerCap).toBeGreaterThan(0)
     await input.fill('')
 
diff --git a/apps/web/tests/composer-draft-scroll.e2e.ts b/apps/web/tests/composer-draft-scroll.e2e.ts
index 34c552401e..f9fa6dd81d 100644
--- a/apps/web/tests/composer-draft-scroll.e2e.ts
+++ b/apps/web/tests/composer-draft-scroll.e2e.ts
@@ -1,36 +1,31 @@
 // Web e2e scenario: a composer draft longer than the 14-line cap scrolls its
-// GLYPHS, not just its caret.
+// GLYPHS AND ITS CARET AS ONE.
 //
 // The composer paints its text in two stacked layers (see
 // packages/client/ui-conversation/src/client/skeleton/InputBar.module.css): the
 // `<textarea>` carries the value, the selection and the caret but renders its
 // own glyphs `color: transparent`, and every visible character is painted by the
 // `[data-input-backdrop]` div underneath it, which also carries the claim-token
-// highlight, the chips and the ghost hint. The backdrop is `position: absolute;
-// inset: 0; overflow: hidden` — it is CLIPPED, not scrolled, and nothing in the
-// browser links its scroll offset to the textarea's.
+// highlight, the chips and the ghost hint.
 //
-// So past the cap the textarea scrolled and the words did not: the caret walked
-// off the bottom of a block of text frozen at line 1, and no gesture — wheel,
-// drag, arrow key — moved it. `InputBar` now mirrors the offset onto the
-// backdrop on every textarea `scroll`, which is the one event every way of
-// moving the box ends in.
+// Two layers can only stay together by moving together. They now do: both sit
+// inside `[data-input-scroll]`, the composer's single scrolling box, and are as
+// tall as the whole draft — so one offset, applied by the browser, moves the
+// caret and the words in the same frame. Scrolling the textarea and assigning
+// its offset to the backdrop looks equivalent and is not: a wheel gesture is
+// composited off the main thread, so the assignment lands frames late and the
+// caret visibly flies ahead of the text it belongs to.
 //
-// Mirroring an offset is only correct while both layers can reach it, so the
-// geometry underneath is asserted here alongside the visible outcome: the
-// backdrop's trailing-line sentinel (a textarea reserves a line box for the
-// caret after a final newline; `pre-wrap` collapses one), and one wrap width
-// across all three layers (only the textarea scrolls, so only it can lose
-// width to a scrollbar that consumes layout space). Either breaks the extent
-// equality, and an unreachable offset clamps the glyphs below the caret.
+// That failure is what the same-task measurement below pins. Every metric here
+// is read through the caret's own coordinate frame — where the textarea puts
+// line n — against where the backdrop paints line n, because that difference is
+// the defect a user sees, and it is the one number a mirror between two boxes
+// cannot hold at zero.
 //
-// Only a real engine can show this. Scrolling is layout: jsdom reports
+// Only a real engine can show any of this. Scrolling is layout: jsdom reports
 // `scrollHeight === clientHeight` for every element and never scrolls one, so
-// the unit spec in packages/client/ui-conversation/tests/input-bar.spec.tsx has
-// to stub both offsets and can only prove the mirroring code path runs. What is
-// asserted here instead is the user-visible fact that path exists for — after
-// scrolling to the end of a long draft, the LAST line is the one on screen —
-// measured with a DOM Range over the backdrop's own text.
+// the unit spec in packages/client/ui-conversation/tests/input-bar.spec.tsx can
+// only assert that one scrollport contains both layers.
 //
 // Zero model calls: a fresh workspace's blank session already carries a live
 // composer, and the scenario only types into it. A stray stream would fail loud
@@ -49,10 +44,10 @@ import { connectFreshWorkspace, newEnglishPage, saveFailureShot } from './suppor
 const SNAPSHOT_DIR = fileURLToPath(new URL('./snapshots/composer-draft-scroll', import.meta.url))
 /**
  * Committed golden of the composer's two-layer scroll geometry. The change
- * alters no DOM and no accessible name, so the aria goldens the other scenarios
- * commit are byte-identical with and without it; this records the relations
- * instead, which makes a shift in the cap or in the layer coupling a reviewable
- * diff rather than an assertion someone has to reconstruct.
+ * alters no accessible name, so the aria goldens the other scenarios commit are
+ * byte-identical with and without it; this records the relations instead, which
+ * makes a shift in the cap or in the layer coupling a reviewable diff rather
+ * than an assertion someone has to reconstruct.
  */
 const GEOMETRY_EXPECTED = join(SNAPSHOT_DIR, 'geometry.expected.md')
 const MODE = webSnapshotMode()
@@ -69,41 +64,54 @@ const DRAFT = Array.from({ length: DRAFT_LINES }, (_unused, index) => {
 }).join('\n')
 
 /**
- * A draft ending in a newline: the shape whose layer extents diverge without
- * the backdrop's trailing-line sentinel. A textarea reserves a line box for the
- * caret after a final newline; `white-space: pre-wrap` collapses a text node's
- * trailing newline and generates none, so the backdrop would come out exactly
- * one line shorter and the mirrored offset would clamp a line above the caret.
+ * A draft ending in a newline: the shape where the two layers reserve their
+ * final line box on different terms. A textarea keeps one for the caret after a
+ * final newline; `white-space: pre-wrap` collapses a text node's trailing
+ * newline and generates none. The hidden auto-grow mirror carries the newline
+ * and so decides the height for both, which is why the backdrop needs no
+ * padding of its own — but only a draft of this shape can show it.
  */
 const DRAFT_TRAILING_NEWLINE = `${DRAFT}\n`
 
-/** The composer's two text layers as the browser lays them out. */
+/** The composer's text layers as the browser lays them out. */
 interface ComposerMetrics {
   /** True when the draft is taller than the capped box — the situation under test. */
   overflows: boolean
-  /** Visible height of the textarea's content box: the cap in pixels. */
+  /** Visible height of the scrollport's content box: the cap in pixels. */
   clientHeight: number
   /** Whole lines that fit in the visible box, at the composer's own line-height. */
   visibleLines: number
-  /** The textarea's scroll offset, which the caret and the selection follow. */
-  inputScrollTop: number
-  /** The backdrop's scroll offset, which every visible glyph follows. */
-  backdropScrollTop: number
-  /** True when the two layers agree — the coupling this scenario exists for. */
-  layersAgree: boolean
+  /** The composer's one scroll offset, which the caret and the glyphs both follow. */
+  scrollTop: number
+  /** Furthest that offset can go. */
+  scrollMax: number
+  /**
+   * Scrollable overflow the textarea holds on its own — 0, or a second offset
+   * exists that nothing keeps equal to this one.
+   */
+  inputScrollable: number
+  /**
+   * Distance between where the caret sits for a draft line and where the
+   * backdrop paints that line, in pixels. A fixed value (the difference between
+   * a line box's top and its glyph box's) is alignment; a value that CHANGES
+   * with the scroll offset is the defect — the words trailing the caret.
+   */
+  caretGlyphGap: number
+  /**
+   * How much that gap moves when the offset changes inside a single task: 0
+   * here, because one box carries both layers. Assigning one box's offset to
+   * another cannot be 0 — a scroll event is dispatched after the task that
+   * moved the box, so between the two there is a frame with the caret at the
+   * new offset and the glyphs at the old one.
+   */
+  gapShiftOnScroll: number
   /**
    * Top of the LAST draft line relative to the visible box's top, in pixels: at
-   * most `clientHeight` when that line is on screen. This is the reported
-   * symptom as a number — with the layers uncoupled the backdrop stays at offset
-   * 0, so the last line sits a full draft-height below the box.
+   * most `clientHeight` when that line is on screen.
    */
   lastLineOffset: number
   /** Top of the FIRST draft line relative to the visible box's top: negative once it has scrolled out. */
   firstLineOffset: number
-  /** Furthest the textarea can scroll. */
-  inputMax: number
-  /** Furthest the backdrop can scroll — equal to `inputMax`, or the mirror clamps below the caret. */
-  backdropMax: number
   /** Content width the textarea wraps at. */
   inputWrapWidth: number
   /** Content width the backdrop wraps at — equal, or the layers break lines in different places. */
@@ -113,14 +121,16 @@ interface ComposerMetrics {
 }
 
 /**
- * Measure both composer layers in the page.
+ * Measure the composer's layers in the page, in the caret's coordinate frame.
  * @param page - the page under test.
- * @returns the two layers' offsets and where the draft's first and last lines sit.
+ * @returns the offset, the caret-to-glyph gap, and where the draft's first and last lines sit.
  */
 function measureComposer(page: Page): Promise<ComposerMetrics> {
   return page.evaluate(({ first, last }) => {
     const input = document.querySelector<HTMLTextAreaElement>('textarea:enabled')
     if (input === null) throw new Error('no live composer textarea in the DOM')
+    const scroll = input.closest<HTMLElement>('[data-input-scroll]')
+    if (scroll === null) throw new Error('the composer textarea is not inside a draft scrollport')
     const backdrop = input.parentElement?.querySelector<HTMLElement>('[data-input-backdrop]')
     if (backdrop === undefined || backdrop === null) throw new Error('no decoration backdrop beside the composer textarea')
     // The hidden auto-grow mirror: the textarea's next sibling, and the layer
@@ -128,47 +138,50 @@ function measureComposer(page: Page): Promise<ComposerMetrics> {
     // two that carry glyphs.
     const mirror = input.nextElementSibling
     if (!(mirror instanceof HTMLElement)) throw new Error('no auto-grow mirror after the composer textarea')
-    const box = input.getBoundingClientRect()
     // The draft carries no chips or claim token, so the decoration walk emits it
-    // as one text node — the backdrop's first, ahead of the trailing-line
-    // sentinel React renders as a second one. Both markers live in that first
-    // node, which is what the Range below needs.
+    // as a single text node, which is what the Range below needs.
     const text = backdrop.firstChild
     if (!(text instanceof Text)) throw new Error('backdrop does not open with a plain text node')
-    const offsetOf = (marker: string): number => {
+    const lineHeight = Number.parseFloat(getComputedStyle(input).lineHeight)
+    /** Where the backdrop paints the line holding `marker`, in viewport coordinates. */
+    const glyphTop = (marker: string): number => {
       const at = text.data.indexOf(marker)
       if (at < 0) throw new Error(`marker ${marker} missing from the backdrop text`)
       const range = document.createRange()
       range.setStart(text, at)
       range.setEnd(text, at + marker.length)
-      return range.getBoundingClientRect().top - box.top
+      return range.getBoundingClientRect().top
     }
-    const lineHeight = Number.parseFloat(getComputedStyle(input).lineHeight)
-    // Each layer's own maximum, probed by asking for an impossible offset and
-    // reading back what it clamped to, then restored. Reading scrollHeight -
-    // clientHeight instead would compute the maximum rather than observe it.
-    const restore = input.scrollTop
-    const restoreBackdrop = backdrop.scrollTop
-    input.scrollTop = 1e7
-    backdrop.scrollTop = 1e7
-    const inputMax = input.scrollTop
-    const backdropMax = backdrop.scrollTop
-    input.scrollTop = restore
-    backdrop.scrollTop = restoreBackdrop
+    const paddingTop = Number.parseFloat(getComputedStyle(input).paddingTop)
+    // Where the CARET sits on the draft's first line: the textarea lays its own
+    // (transparent) glyphs out from its border box, shifted by any offset it
+    // holds itself. Reading the caret's frame this way rather than the
+    // scrollport's is what makes the gap the user-visible quantity — it stays
+    // honest if the textarea ever starts scrolling on its own again.
+    const gap = (): number =>
+      Math.round(input.getBoundingClientRect().top + paddingTop - input.scrollTop - glyphTop(first))
+    // The same-task probe: move the offset and re-read the gap before the task
+    // ends, which is before any scroll event could have run a listener.
+    const before = gap()
+    const restore = scroll.scrollTop
+    scroll.scrollTop = restore === 0 ? 120 : 0
+    const gapShiftOnScroll = Math.abs(gap() - before)
+    scroll.scrollTop = restore
+    const box = scroll.getBoundingClientRect()
     return {
-      inputMax,
-      backdropMax,
       inputWrapWidth: input.clientWidth,
       backdropWrapWidth: backdrop.clientWidth,
       mirrorWrapWidth: mirror.clientWidth,
-      overflows: input.scrollHeight > input.clientHeight,
-      clientHeight: input.clientHeight,
-      visibleLines: Math.floor(input.clientHeight / lineHeight),
-      inputScrollTop: input.scrollTop,
-      backdropScrollTop: backdrop.scrollTop,
-      layersAgree: input.scrollTop === backdrop.scrollTop,
-      lastLineOffset: offsetOf(last),
-      firstLineOffset: offsetOf(first),
+      overflows: scroll.scrollHeight > scroll.clientHeight,
+      clientHeight: scroll.clientHeight,
+      visibleLines: Math.floor(scroll.clientHeight / lineHeight),
+      scrollTop: scroll.scrollTop,
+      scrollMax: scroll.scrollHeight - scroll.clientHeight,
+      inputScrollable: input.scrollHeight - input.clientHeight,
+      caretGlyphGap: before,
+      gapShiftOnScroll,
+      lastLineOffset: glyphTop(last) - box.top,
+      firstLineOffset: glyphTop(first) - box.top,
     }
   }, { first: FIRST_MARKER, last: LAST_MARKER })
 }
@@ -179,9 +192,9 @@ function measureComposer(page: Page): Promise<ComposerMetrics> {
  * Absolute glyph coordinates are deliberately absent: they depend on font
  * metrics and would make the fixture fail on a machine that measures text
  * differently — a golden that needs re-recording per platform documents the
- * platform, not the change. What is recorded is the cap, the layer agreement,
- * and which lines are on screen, each a comparison that survives any layout
- * keeping the coupling.
+ * platform, not the change. What is recorded is the cap, the caret-to-glyph
+ * relation, and which lines are on screen, each a comparison that survives any
+ * layout keeping the coupling.
  * @param top - metrics with the draft scrolled to its start.
  * @param bottom - metrics with the draft scrolled to its end.
  * @param trailingNewline - metrics with the trailing-newline draft scrolled to its end.
@@ -189,33 +202,35 @@ function measureComposer(page: Page): Promise<ComposerMetrics> {
  */
 function renderGeometry(top: ComposerMetrics, bottom: ComposerMetrics, trailingNewline: ComposerMetrics): string {
   return [
-    '# Composer draft scrolling (14-line cap, two text layers)',
+    '# Composer draft scrolling (14-line cap, two text layers, one scrollport)',
     '',
     '## At the start of the draft',
     '',
     `- draft overflows the capped box: ${String(top.overflows)}`,
     `- visible lines: ${String(top.visibleLines)}`,
-    `- both layers share one scroll extent: ${String(top.inputMax === top.backdropMax)}`,
+    `- the textarea holds no scroll offset of its own: ${String(top.inputScrollable === 0)}`,
     `- all three layers wrap at one width: ${String(
       top.inputWrapWidth === top.backdropWrapWidth && top.backdropWrapWidth === top.mirrorWrapWidth,
     )}`,
-    `- textarea scroll offset: ${String(top.inputScrollTop)}px`,
-    `- glyph layer tracks it: ${String(top.layersAgree)}`,
+    `- scroll offset: ${String(top.scrollTop)}px`,
+    `- caret and glyphs stay level when the offset changes: ${String(top.gapShiftOnScroll === 0)}`,
     `- first draft line is on screen: ${String(top.firstLineOffset >= 0 && top.firstLineOffset < top.clientHeight)}`,
     `- last draft line is on screen: ${String(top.lastLineOffset >= 0 && top.lastLineOffset < top.clientHeight)}`,
     '',
     '## Scrolled to the end of the draft',
     '',
-    `- textarea moved: ${String(bottom.inputScrollTop > 0)}`,
-    `- glyph layer tracks it: ${String(bottom.layersAgree)}`,
+    `- offset moved: ${String(bottom.scrollTop > 0)}`,
+    `- caret sits on its own glyphs: ${String(bottom.caretGlyphGap === top.caretGlyphGap)}`,
+    `- caret and glyphs stay level when the offset changes: ${String(bottom.gapShiftOnScroll === 0)}`,
     `- first draft line has scrolled out above: ${String(bottom.firstLineOffset < 0)}`,
     `- last draft line is on screen: ${String(bottom.lastLineOffset >= 0 && bottom.lastLineOffset < bottom.clientHeight)}`,
     '',
     '## Draft ending in a newline, scrolled to the end',
     '',
-    `- both layers share one scroll extent: ${String(trailingNewline.inputMax === trailingNewline.backdropMax)}`,
-    `- glyph layer tracks the caret: ${String(trailingNewline.layersAgree)}`,
-    `- last draft line is on screen: ${String(trailingNewline.lastLineOffset >= 0 && trailingNewline.lastLineOffset < trailingNewline.clientHeight)}`,
+    `- caret sits on its own glyphs: ${String(trailingNewline.caretGlyphGap === top.caretGlyphGap)}`,
+    `- the draft's own last line is on screen: ${String(
+      trailingNewline.lastLineOffset >= 0 && trailingNewline.lastLineOffset < trailingNewline.clientHeight,
+    )}`,
   ].join('\n').trimEnd()
 }
 
@@ -251,17 +266,16 @@ describe('web e2e: composer draft scrolling', () => {
     // case below.
     await page.locator('textarea:enabled').first().hover()
     await page.mouse.wheel(0, -2000)
-    await expect.poll(async () => (await measureComposer(page)).inputScrollTop, { timeout: 10_000 }).toBe(0)
+    await expect.poll(async () => (await measureComposer(page)).scrollTop, { timeout: 10_000 }).toBe(0)
     const metrics = await measureComposer(page)
     // The cap is the composer seat's `--dsh-composer-text-max-height` (336px =
     // 14 x 24px lines). The count, not the pixels: it is the figma constant and
     // survives a device-pixel-ratio change.
     expect(metrics.visibleLines).toBe(14)
-    // Resting state: the draft's head is what a 40-line draft shows, and its
-    // tail is far below the box. Both layers sit at the origin, which is why the
-    // uncoupled build looks correct until something scrolls.
-    expect(metrics.inputScrollTop).toBe(0)
-    expect(metrics.layersAgree).toBe(true)
+    // One scrolling box: the textarea is as tall as the draft, so there is no
+    // second offset for the caret to hold while the glyphs hold another.
+    expect(metrics.inputScrollable).toBe(0)
+    expect(metrics.scrollTop).toBe(0)
     expect(metrics.firstLineOffset).toBeGreaterThanOrEqual(0)
     expect(metrics.firstLineOffset).toBeLessThan(metrics.clientHeight)
     expect(metrics.lastLineOffset).toBeGreaterThan(metrics.clientHeight)
@@ -270,19 +284,12 @@ describe('web e2e: composer draft scrolling', () => {
 
   it('lays out all three text layers at one wrap width', async () => {
     onTestFailed(() => saveFailureShot(page, 'web-e2e-composer-draft-scroll-wrap-width'))
-    // The premise under the mirror, asserted rather than assumed. Only .input
-    // scrolls, so only .input can lose content width to a scrollbar that
-    // consumes layout space; a narrower .input wraps a long draft onto more
-    // lines, ends up taller, and its larger maximum makes the mirrored offset
-    // clamp below the caret. Measured on a standalone harness, an 8px width
-    // difference is worth 2 to 5 lines on a wrap-sensitive draft.
-    //
-    // This holds on the lane's engine and is what a regression would break —
-    // it is NOT vacuous: measured on the same app, WebKit reports 768 against
-    // 776 here, which is the divergence the Agent Note records as a
-    // pre-existing, engine-specific limitation. The mirror is unaffected there
-    // today because the extents still agree; this assertion is what would
-    // notice if the lane's engine ever moved into the same state.
+    // A layer that breaks lines somewhere else puts the words under the wrong
+    // caret, and an 8px difference is worth 2 to 5 lines on a wrap-sensitive
+    // draft. The three now share a containing block — the scrollport — so a
+    // scrollbar that consumes layout space costs them the same width; before,
+    // only the textarea scrolled, and WebKit reserved gutter space for it alone
+    // (768 against 776) while chromium and firefox did not.
     const metrics = await measureComposer(page)
     expect(metrics.backdropWrapWidth).toBe(metrics.inputWrapWidth)
     // The mirror decides the box height, so it belongs in the same equality —
@@ -292,41 +299,55 @@ describe('web e2e: composer draft scrolling', () => {
     expect(tripwire.pageErrors).toEqual([])
   }, 60_000)
 
+  it('the glyphs cannot lag the caret: one task moves both', async () => {
+    onTestFailed(() => saveFailureShot(page, 'web-e2e-composer-draft-scroll-lag'))
+    // The reported symptom, isolated. A scroll offset changes and the caret's
+    // distance to its own glyphs is re-read before the task ends — before any
+    // `scroll` listener could have run. With the layers on one scrollport the
+    // browser moved both, so the distance is unchanged; with the glyph layer
+    // catching up in a listener it is off by the whole delta until a later
+    // frame, which is a caret flying away from its text mid-gesture.
+    const metrics = await measureComposer(page)
+    expect(metrics.gapShiftOnScroll).toBe(0)
+    expect(tripwire.pageErrors).toEqual([])
+  }, 60_000)
+
   it('a wheel gesture over a long draft moves the words, not only the caret', async () => {
     onTestFailed(() => saveFailureShot(page, 'web-e2e-composer-draft-scroll-wheel'))
     const input = page.locator('textarea:enabled').first()
     await input.hover()
-    // One delta past the whole draft: the textarea clamps at its own end, and
-    // the wheel-chaining handler leaves it native because the box is not yet at
-    // its edge when the gesture starts (the chaining itself is owned by the
-    // unit spec).
+    const resting = (await measureComposer(page)).caretGlyphGap
+    // One delta past the whole draft: the box clamps at its own end, and the
+    // wheel-chaining handler leaves it native because the box is not yet at its
+    // edge when the gesture starts (the chaining itself is owned by the unit spec).
     await page.mouse.wheel(0, 2000)
-    await expect.poll(async () => (await measureComposer(page)).inputScrollTop, { timeout: 10_000 })
+    await expect.poll(async () => (await measureComposer(page)).scrollTop, { timeout: 10_000 })
       .toBeGreaterThan(0)
     const metrics = await measureComposer(page)
-    // The coupling, stated directly.
-    expect(metrics.layersAgree).toBe(true)
+    // The caret is still on its own glyphs after the gesture.
+    expect(metrics.caretGlyphGap).toBe(resting)
     // The reported symptom, stated as what the user sees: the end of the draft
-    // is on screen and its beginning is not. On the uncoupled build the glyph
-    // layer stays at offset 0, so `lastLineOffset` is still a full draft below
-    // the box and `firstLineOffset` is still 0 — the text never moved.
+    // is on screen and its beginning is not.
     expect(metrics.lastLineOffset).toBeGreaterThanOrEqual(0)
     expect(metrics.lastLineOffset).toBeLessThan(metrics.clientHeight)
     expect(metrics.firstLineOffset).toBeLessThan(0)
     expect(tripwire.pageErrors).toEqual([])
   }, 60_000)
 
-  it('typing at the end of a scrolled draft keeps the layers together', async () => {
+  it('typing at the end of a scrolled draft brings the caret back into view', async () => {
     onTestFailed(() => saveFailureShot(page, 'web-e2e-composer-draft-scroll-edit'))
-    // The other way the box moves. Typing at the caret — parked at the draft's
-    // end by the wheel gesture — scrolls it into view, which is a `scroll` like
-    // any other; this pins that an edit is not a separate case needing its own
-    // mirror, which is why one listener is the whole implementation.
+    // The other way the box moves, and the one that depends on the browser: the
+    // textarea no longer scrolls, so revealing the caret after an edit is a
+    // scroll-into-view that has to walk up to the scrollport. Scroll away from
+    // the caret first, so the edit has somewhere to bring it back from.
     const input = page.locator('textarea:enabled').first()
     await input.press('End')
+    await input.hover()
+    await page.mouse.wheel(0, -2000)
+    await expect.poll(async () => (await measureComposer(page)).scrollTop, { timeout: 10_000 }).toBe(0)
     await input.pressSequentially(' tail')
     const metrics = await measureComposer(page)
-    expect(metrics.layersAgree).toBe(true)
+    expect(metrics.scrollTop).toBeGreaterThan(0)
     expect(metrics.lastLineOffset).toBeGreaterThanOrEqual(0)
     expect(metrics.lastLineOffset).toBeLessThan(metrics.clientHeight)
     expect(tripwire.pageErrors).toEqual([])
@@ -335,23 +356,21 @@ describe('web e2e: composer draft scrolling', () => {
   it('a draft ending in a newline scrolls to its true end, not a line above it', async () => {
     onTestFailed(() => saveFailureShot(page, 'web-e2e-composer-draft-scroll-trailing-newline'))
     // The layers reserve a final line box on different terms, so this shape is
-    // the one that separates equal extents from a mirror that clamps early.
+    // the one that separates a height every layer agrees on from a box measured
+    // one line short of the caret's own last position.
     const input = page.locator('textarea:enabled').first()
     await input.fill(DRAFT_TRAILING_NEWLINE)
     await expect.poll(async () => (await measureComposer(page)).overflows, { timeout: 10_000 }).toBe(true)
-    const extents = await measureComposer(page)
-    // The invariant the sentinel exists for. Without it the textarea measured
-    // 652 against the backdrop's 628 — one 24px line apart.
-    expect(extents.backdropMax).toBe(extents.inputMax)
     await input.hover()
     await page.mouse.wheel(0, 4000)
     await expect.poll(async () => {
       const m = await measureComposer(page)
-      return m.inputScrollTop === m.inputMax
+      return m.scrollTop === m.scrollMax
     }, { timeout: 10_000 }).toBe(true)
     const bottom = await measureComposer(page)
-    // At the very bottom the glyphs are level with the caret, not a line behind.
-    expect(bottom.layersAgree).toBe(true)
+    // At the very bottom the glyphs are level with the caret, and the draft's
+    // own last line — the one before the empty final line — is on screen.
+    expect(bottom.gapShiftOnScroll).toBe(0)
     expect(bottom.lastLineOffset).toBeGreaterThanOrEqual(0)
     expect(bottom.lastLineOffset).toBeLessThan(bottom.clientHeight)
     expect(tripwire.pageErrors).toEqual([])
@@ -365,11 +384,11 @@ describe('web e2e: composer draft scrolling', () => {
     await input.fill(DRAFT)
     await input.hover()
     await page.mouse.wheel(0, -2000)
-    await expect.poll(async () => (await measureComposer(page)).inputScrollTop, { timeout: 10_000 }).toBe(0)
+    await expect.poll(async () => (await measureComposer(page)).scrollTop, { timeout: 10_000 }).toBe(0)
     const top = await measureComposer(page)
     await input.hover()
     await page.mouse.wheel(0, 2000)
-    await expect.poll(async () => (await measureComposer(page)).inputScrollTop, { timeout: 10_000 })
+    await expect.poll(async () => (await measureComposer(page)).scrollTop, { timeout: 10_000 })
       .toBeGreaterThan(0)
     const bottom = await measureComposer(page)
     await input.fill(DRAFT_TRAILING_NEWLINE)
@@ -377,7 +396,7 @@ describe('web e2e: composer draft scrolling', () => {
     await page.mouse.wheel(0, 4000)
     await expect.poll(async () => {
       const m = await measureComposer(page)
-      return m.inputScrollTop === m.inputMax
+      return m.scrollTop === m.scrollMax
     }, { timeout: 10_000 }).toBe(true)
     const trailingNewline = await measureComposer(page)
     await compareOrRefreshGolden(GEOMETRY_EXPECTED, renderGeometry(top, bottom, trailingNewline), MODE)
diff --git a/apps/web/tests/snapshots/composer-draft-scroll/geometry.expected.md b/apps/web/tests/snapshots/composer-draft-scroll/geometry.expected.md
index c15d0f0350..51191565df 100644
--- a/apps/web/tests/snapshots/composer-draft-scroll/geometry.expected.md
+++ b/apps/web/tests/snapshots/composer-draft-scroll/geometry.expected.md
@@ -1,25 +1,25 @@
-# Composer draft scrolling (14-line cap, two text layers)
+# Composer draft scrolling (14-line cap, two text layers, one scrollport)
 
 ## At the start of the draft
 
 - draft overflows the capped box: true
 - visible lines: 14
-- both layers share one scroll extent: true
+- the textarea holds no scroll offset of its own: true
 - all three layers wrap at one width: true
-- textarea scroll offset: 0px
-- glyph layer tracks it: true
+- scroll offset: 0px
+- caret and glyphs stay level when the offset changes: true
 - first draft line is on screen: true
 - last draft line is on screen: false
 
 ## Scrolled to the end of the draft
 
-- textarea moved: true
-- glyph layer tracks it: true
+- offset moved: true
+- caret sits on its own glyphs: true
+- caret and glyphs stay level when the offset changes: true
 - first draft line has scrolled out above: true
 - last draft line is on screen: true
 
 ## Draft ending in a newline, scrolled to the end
 
-- both layers share one scroll extent: true
-- glyph layer tracks the caret: true
-- last draft line is on screen: true
+- caret sits on its own glyphs: true
+- the draft's own last line is on screen: true
diff --git a/packages/client/ui-conversation/src/client/skeleton/InputBar.module.css b/packages/client/ui-conversation/src/client/skeleton/InputBar.module.css
index 1badf3091f..939176092f 100644
--- a/packages/client/ui-conversation/src/client/skeleton/InputBar.module.css
+++ b/packages/client/ui-conversation/src/client/skeleton/InputBar.module.css
@@ -88,11 +88,11 @@
   box-shadow: var(--dsw-shadow-lv2);
   font-size: 16px;
   line-height: 24px;
-  /* Elevated surface in dark, same as the menus: the textarea inside scrolls
-     once the composer hits its height cap, so the thumb takes the l2 pair.
-     Declared on the card because the elevation belongs to the surface, and the
-     custom properties inherit down to the textarea that actually scrolls (see
-     ui-theme styles/scrollbar.css for the rebinding contract). */
+  /* Elevated surface in dark, same as the menus: the draft scrollport inside
+     scrolls once the composer hits its height cap, so the thumb takes the l2
+     pair. Declared on the card because the elevation belongs to the surface,
+     and the custom properties inherit down to the box that actually scrolls
+     (see ui-theme styles/scrollbar.css for the rebinding contract). */
   --dsh-scrollbar-thumb: var(--dsw-alias-scrollbar-bg-l2);
   --dsh-scrollbar-thumb-hover: var(--dsw-alias-scrollbar-hover-l2);
 }
@@ -112,8 +112,21 @@
   height: 0;
 }
 
-/* Mirror-div auto-grow wrapper: the hidden mirror is in normal flow and sets the height
-   (min 2 lines / max 14 lines); the textarea rides it absolutely. Mirror and textarea
+/* The draft's scrollport, and the ONLY scrolling box in the composer: the
+   caret is the textarea's and every visible glyph is the backdrop's, so the two
+   layers stay together only by riding one offset the browser applies to both at
+   once. Scrolling one box and assigning the offset to the other cannot hold —
+   a wheel gesture is composited off the main thread, so the assignment lands
+   frames late and the words visibly trail the caret. The 14-line cap lives here
+   because this is the box the cap describes. */
+.scroll {
+  max-height: var(--dsh-composer-text-max-height);
+  overflow-y: auto;
+}
+
+/* Mirror-div auto-grow stack: the hidden mirror is in normal flow and sets the FULL draft
+   height (min 2 lines in hero); backdrop and textarea ride it absolutely, so both layers are
+   as tall as the draft and the scrollport above shows a window onto them. Mirror and textarea
    MUST share font, line-height, padding and wrapping rules or heights diverge. */
 .grow {
   position: relative;
@@ -165,7 +178,11 @@
   width: 100%;
   height: 100%;
   resize: none;
-  overflow-y: auto;
+  /* Never a scroller of its own: it is as tall as the draft, so it has no
+     scrollable overflow to hold an offset that could differ from the glyphs'.
+     The browser still reveals the caret — the scroll-into-view walks up to
+     .scroll and moves both layers together. */
+  overflow: hidden;
   border: none;
   outline: none;
   background: transparent;
@@ -193,18 +210,16 @@
   white-space: pre-wrap;
   word-break: break-word;
   overflow-wrap: anywhere;
-  /* These three MUST wrap at one width, because InputBar mirrors a single
-     scroll offset between .input and .backdrop and a layer that wraps onto
-     more lines is taller, has a larger scroll maximum, and clamps the mirrored
-     offset below the caret. Only .input scrolls, so only .input can lose
-     content width to a scrollbar that consumes layout space.
-     `scrollbar-gutter: stable` here does NOT buy that guarantee and was
-     removed after measuring: WebKit applies it to overflow-y:auto but not to
-     the overflow:hidden layers, so it left .input at 768 against 776 — the
-     same gap it was meant to close — while costing chromium 8px of text width
-     unconditionally. The gap it would have closed is measured and recorded in
-     the Agent Note (2026-07-31-composer-glyph-layer-tracks-the-textarea);
-     closing it needs one geometry every engine agrees on, not this property. */
+  /* These three MUST wrap at one width: the mirror decides the box height
+     the other two are laid out in, and a glyph layer that breaks lines
+     elsewhere than the textarea puts the words under the wrong caret. They do
+     so by construction now that all three sit INSIDE .scroll — a scrollbar
+     that consumes layout space narrows the scrollport, which is their shared
+     containing block, so it costs all three the same width on every engine.
+     Scrolling the textarea itself is what used to break this, and no property
+     fixed it: WebKit reserved gutter space for the overflow-y:auto textarea
+     and not for the overflow:hidden layers beside it, leaving them 8px apart
+     (768 against 776) — worth 2 to 5 wrapped lines on a long draft. */
 }
 
 /* figma 34:10434: #ADB2B8 light / #81858C dark — the caption pair exactly. */
@@ -222,10 +237,6 @@
 .mirror {
   visibility: hidden;
   pointer-events: none;
-  /* 14-line cap, shared with the composer takeovers (declared on
-     ConversationRoot .composerSeat). */
-  max-height: var(--dsh-composer-text-max-height);
-  overflow: hidden;
 }
 
 /* Hero (centered empty-state) keeps the 2-line floor (figma min-h 52 = ~2 × 24
diff --git a/packages/client/ui-conversation/src/client/skeleton/InputBar.tsx b/packages/client/ui-conversation/src/client/skeleton/InputBar.tsx
index bc762ab7f3..3610b9143c 100644
--- a/packages/client/ui-conversation/src/client/skeleton/InputBar.tsx
+++ b/packages/client/ui-conversation/src/client/skeleton/InputBar.tsx
@@ -62,7 +62,7 @@ export function InputBar({
   const draft = input?.draft ?? ''
   const empty = draft.trim() === ''
   const inputRef = useRef<HTMLTextAreaElement | null>(null)
-  const backdropRef = useRef<HTMLDivElement | null>(null)
+  const scrollRef = useRef<HTMLDivElement | null>(null)
   // IME guard: composition Enter picks a candidate, it must not send. The ref outlives renders;
   // clearing is deferred one tick because Safari delivers the closing keydown AFTER compositionend.
   const composingRef = useRef(false)
@@ -92,24 +92,14 @@ export function InputBar({
     if (!locked) inputRef.current?.focus()
   }, [locked, sessionId])
 
-  // Two DOM listeners on the textarea, one lifetime (it is never unmounted —
-  // the inert state renders the same element disabled).
-  //
-  // wheel — active conversation scrollport: chain the gesture. While the
-  // textarea (capped at 14 lines with overflow-y:auto) can still move in this
-  // direction, keep the native scroll; only at its own edge forward delta to
-  // the host so a short draft never traps the gesture and a long draft stays
-  // scrollable. Hero mounts have no host and keep native wheel scrolling.
-  //
-  // scroll — the backdrop paints every visible glyph (the textarea's own text
-  // is transparent) but is clipped, not scrolled, so it does not follow the
-  // textarea on its own: without this mirror a draft past the cap moves the
-  // caret while the words stay frozen in place. Every way the box moves ends
-  // in a `scroll` event, edits included (the caret is scrolled into view), and
-  // the layers share an extent, so a draft that shrinks past the offset clamps
-  // both to the same maximum — one listener covers the coupling.
+  // Wheel chaining on the draft scrollport, one lifetime (it is never
+  // unmounted — the inert state renders the same element disabled). While the
+  // capped box can still move in this direction, keep the native scroll; only
+  // at its own edge forward the delta to the active conversation scrollport, so
+  // a short draft never traps the gesture and a long draft stays scrollable.
+  // Hero mounts have no host and keep native wheel scrolling.
   useEffect(() => {
-    const el = inputRef.current
+    const el = scrollRef.current
     if (el === null) return
     const onWheel = (e: WheelEvent): void => {
       const host = el.closest('[data-conversation-scroll]')
@@ -120,16 +110,8 @@ export function InputBar({
       e.preventDefault()
       host.scrollTop += e.deltaY
     }
-    const onScroll = (): void => {
-      const backdropEl = backdropRef.current
-      if (backdropEl !== null) backdropEl.scrollTop = el.scrollTop
-    }
     el.addEventListener('wheel', onWheel, { passive: false })
-    el.addEventListener('scroll', onScroll, { passive: true })
-    return () => {
-      el.removeEventListener('wheel', onWheel)
-      el.removeEventListener('scroll', onScroll)
-    }
+    return () => { el.removeEventListener('wheel', onWheel) }
   }, [])
 
   const onKeyDown = (e: KeyboardEvent<HTMLTextAreaElement>): void => {
@@ -375,22 +357,6 @@ export function InputBar({
       const displayHint = translated !== hintKey ? translated : deco.hint
       backdrop.push(<span key="hint" className={css.hint} data-decoration="hint">{displayHint}</span>)
     }
-    // Trailing-line sentinel, the same one the mirror div carries and for the
-    // same reason: a textarea reserves a line box for the caret after a final
-    // newline, while `white-space: pre-wrap` collapses a text node's trailing
-    // newline and generates none. Without it a draft ending in a newline makes
-    // the backdrop exactly one line SHORTER than the textarea, so mirroring the
-    // offset at the very bottom clamps and the glyphs sit a line behind the
-    // caret. The extra newline is absorbed by that same collapse when the draft
-    // does not end in one, so it costs no height in the ordinary case.
-    //
-    // The mirror only fails one way — a backdrop SHORTER than the textarea
-    // clamps the assignment, while a taller one takes every offset exactly and
-    // hides the surplus below the clip. That is why the ghost hint needs no
-    // handling of its own: it can only add content after the draft and before
-    // this sentinel, never remove a line box, so it moves the pair to equal or
-    // to the safe side.
-    backdrop.push('\n')
   }
 
   return (
@@ -408,32 +374,38 @@ export function InputBar({
       <div className={css.card} data-composer-card>
         {overlay !== undefined && <div className={css.overlayAnchor}>{overlay}</div>}
         {accessory !== undefined && <div className={css.accessory}>{accessory}</div>}
-        {/* Mirror-div auto-grow: the hidden mirror renders draft+'\n' and stretches the wrapper
-            (min/max capped in CSS); the absolutely-positioned textarea rides its height. Counting
-            rows by '\n' cannot see soft wraps. */}
-        <div className={css.grow}>
-          <div ref={backdropRef} aria-hidden className={css.backdrop} data-input-backdrop>{backdrop}</div>
-          <textarea
-            ref={inputRef}
-            className={css.input}
-            value={draft}
-            disabled={locked}
-            readOnly={machineBusy}
-            data-phase={input?.phase ?? 'inert'}
-            placeholder={placeholder ?? (disabled
-              ? t('placeholder.unavailable')
-              : planActive ? t('placeholder.plan') : t('placeholder.default'))}
-            rows={2}
-            onChange={onChange}
-            onKeyDown={onKeyDown}
-            onSelect={onSelect}
-            onCopy={(e) => { onCopyOrCut(e, false) }}
-            onCut={(e) => { onCopyOrCut(e, true) }}
-            onPaste={onPaste}
-            onCompositionStart={onCompositionStart}
-            onCompositionEnd={onCompositionEnd}
-          />
-          <div aria-hidden className={css.mirror}>{`${draft}\n`}</div>
+        {/* One scrollport, two text layers. The hidden mirror renders draft+'\n' and stretches the
+            stack to the draft's FULL height (counting rows by '\n' cannot see soft wraps); the
+            absolutely-positioned backdrop and textarea ride that height, and .scroll — capped at 14
+            lines in CSS — is the only thing that scrolls. The caret belongs to the textarea and the
+            glyphs to the backdrop, so they can only stay together by moving together: one scroll
+            offset the browser applies to both layers at once, never a JS mirror between two boxes,
+            which a compositor-driven gesture outruns and leaves the words trailing the caret. */}
+        <div ref={scrollRef} className={css.scroll} data-input-scroll>
+          <div className={css.grow}>
+            <div aria-hidden className={css.backdrop} data-input-backdrop>{backdrop}</div>
+            <textarea
+              ref={inputRef}
+              className={css.input}
+              value={draft}
+              disabled={locked}
+              readOnly={machineBusy}
+              data-phase={input?.phase ?? 'inert'}
+              placeholder={placeholder ?? (disabled
+                ? t('placeholder.unavailable')
+                : planActive ? t('placeholder.plan') : t('placeholder.default'))}
+              rows={2}
+              onChange={onChange}
+              onKeyDown={onKeyDown}
+              onSelect={onSelect}
+              onCopy={(e) => { onCopyOrCut(e, false) }}
+              onCut={(e) => { onCopyOrCut(e, true) }}
+              onPaste={onPaste}
+              onCompositionStart={onCompositionStart}
+              onCompositionEnd={onCompositionEnd}
+            />
+            <div aria-hidden className={css.mirror}>{`${draft}\n`}</div>
+          </div>
         </div>
         <div className={css.row}>
           <div className={css.tools}>
diff --git a/packages/client/ui-conversation/tests/input-bar.spec.tsx b/packages/client/ui-conversation/tests/input-bar.spec.tsx
index dfabcdbdcb..876628d4c7 100644
--- a/packages/client/ui-conversation/tests/input-bar.spec.tsx
+++ b/packages/client/ui-conversation/tests/input-bar.spec.tsx
@@ -239,7 +239,7 @@ describe('running and lock semantics (queue cut 1)', () => {
     expect((textarea).value).toBe('typed')
   })
 
-  it('wheel over a non-overflowing textarea forwards to the conversation host', () => {
+  it('wheel over a non-overflowing draft forwards to the conversation host', () => {
     const host = document.createElement('div')
     host.setAttribute('data-conversation-scroll', '')
     Object.defineProperty(host, 'scrollTop', { value: 40, writable: true, configurable: true })
@@ -255,17 +255,18 @@ describe('running and lock semantics (queue cut 1)', () => {
     }
   })
 
-  it('wheel chains: long drafts scroll inside the textarea until each edge, then the host', () => {
+  it('wheel chains: long drafts scroll inside the draft scrollport until each edge, then the host', () => {
     const host = document.createElement('div')
     host.setAttribute('data-conversation-scroll', '')
     Object.defineProperty(host, 'scrollTop', { value: 40, writable: true, configurable: true })
     const { view, textarea } = bench()
     host.appendChild(view.container)
     document.body.appendChild(host)
-    Object.defineProperty(textarea, 'clientHeight', { value: 100, configurable: true })
-    Object.defineProperty(textarea, 'scrollHeight', { value: 400, configurable: true })
+    const scrollport = view.container.querySelector<HTMLElement>('[data-input-scroll]')!
+    Object.defineProperty(scrollport, 'clientHeight', { value: 100, configurable: true })
+    Object.defineProperty(scrollport, 'scrollHeight', { value: 400, configurable: true })
     let scrollTop = 150
-    Object.defineProperty(textarea, 'scrollTop', {
+    Object.defineProperty(scrollport, 'scrollTop', {
       configurable: true,
       get: () => scrollTop,
       set: (value: number) => { scrollTop = value },
@@ -289,35 +290,20 @@ describe('running and lock semantics (queue cut 1)', () => {
     }
   })
 
-  it('the decoration backdrop tracks the textarea offset (it paints every visible glyph)', () => {
+  it('the caret layer and the glyph layer ride one scrollport', () => {
     const { view, textarea } = bench({ draft: 'line\n'.repeat(40) })
+    const scroll = view.container.querySelector<HTMLElement>('[data-input-scroll]')!
     const backdrop = view.container.querySelector<HTMLElement>('[data-input-backdrop]')!
-    Object.defineProperty(backdrop, 'scrollTop', { value: 0, writable: true, configurable: true })
-    Object.defineProperty(textarea, 'scrollTop', { value: 0, writable: true, configurable: true })
-    // A scrolled draft: the textarea moves, the clipped backdrop must follow.
-    textarea.scrollTop = 120
-    fireEvent.scroll(textarea)
-    expect(backdrop.scrollTop).toBe(120)
-    // Every later move tracks too, including back to the top — a one-shot
-    // mirror would leave the glyphs parked at the first offset it saw.
-    textarea.scrollTop = 0
-    fireEvent.scroll(textarea)
-    expect(backdrop.scrollTop).toBe(0)
-  })
-
-  it('the backdrop carries the trailing-line sentinel that keeps its extent equal to the textarea', () => {
-    // jsdom has no layout, so the HEIGHTS this protects cannot be asserted here
-    // (the browser scenario owns that); what is checkable is that the backdrop's
-    // text is the draft plus exactly one newline. A textarea reserves a line box
-    // after a final newline and `pre-wrap` collapses one, so without the
-    // sentinel a draft ending in a newline leaves the backdrop a line short and
-    // the mirrored offset clamps.
-    const withNewline = bench({ draft: 'alpha\nbeta\n' })
-    const backdrop = withNewline.view.container.querySelector<HTMLElement>('[data-input-backdrop]')!
-    expect(backdrop.textContent).toBe('alpha\nbeta\n\n')
-    const withoutNewline = bench({ draft: 'alpha\nbeta' })
-    const plain = withoutNewline.view.container.querySelector<HTMLElement>('[data-input-backdrop]')!
-    expect(plain.textContent).toBe('alpha\nbeta\n')
+    // The caret is the textarea's and every visible glyph is the backdrop's, so
+    // one box has to carry both or an offset can exist in one and not the other.
+    // jsdom has no layout — the browser scenario owns the geometry; what is
+    // checkable here is that there is exactly one scrolling box and it holds
+    // both layers.
+    expect(scroll.contains(textarea)).toBe(true)
+    expect(scroll.contains(backdrop)).toBe(true)
+    // The glyph layer carries the draft and nothing else: with one scrollport
+    // it no longer pads its own height to match a second box's scroll extent.
+    expect(backdrop.textContent).toBe('line\n'.repeat(40))
   })
 
   it('disabled state shows the unavailable placeholder; custom placeholder wins', () => {