Merge pull request #1039 from deepseek-harness/worktree/composer-scroll-sync

fix(web): scroll the composer's glyph layer with its textarea

.agents/notes/implemented/bug-fix/2026-07-31-composer-glyph-layer-tracks-the-textarea.i18n.yaml

@@ -0,0 +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

.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.

.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。

apps/web/tests/composer-draft-scroll.e2e.ts

@@ -0,0 +1,397 @@
+// Web e2e scenario: a composer draft longer than the 14-line cap scrolls its
+// GLYPHS, not just its caret.
+//
+// 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.
+//
+// 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.
+//
+// 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.
+//
+// Only a real engine can show 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.
+//
+// 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
+// with NO_ADAPTER.
+import { fileURLToPath } from 'node:url'
+import { join } from 'node:path'
+import type { Browser, Page } from 'playwright'
+import { chromium } from 'playwright'
+import { afterAll, beforeAll, describe, expect, it, onTestFailed } from 'vitest'
+import {
+  assertFixtureInventory, compareOrRefreshGolden, launchWebScaffold, watchConsole,
+  webSnapshotMode, type WebScaffold,
+} from './scaffold.ts'
+import { connectFreshWorkspace, newEnglishPage, saveFailureShot } from './support.ts'
+
+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.
+ */
+const GEOMETRY_EXPECTED = join(SNAPSHOT_DIR, 'geometry.expected.md')
+const MODE = webSnapshotMode()
+
+/** Marks the first and last line so a Range can find them in the backdrop's text. */
+const FIRST_MARKER = 'FIRST-LINE-MARKER'
+const LAST_MARKER = 'LAST-LINE-MARKER'
+/** Comfortably past the 14-line cap, so the draft overflows however the lines wrap. */
+const DRAFT_LINES = 40
+const DRAFT = Array.from({ length: DRAFT_LINES }, (_unused, index) => {
+  if (index === 0) return FIRST_MARKER
+  if (index === DRAFT_LINES - 1) return LAST_MARKER
+  return `draft line ${String(index + 1).padStart(2, '0')}`
+}).join('\n')
+
+/**
+ * A draft ending in a newline: the shape whose layer extents diverge without
+ * the backdrop's trailing-line sentinel. A textarea reserves a line box for the
+ * caret after a final newline; `white-space: pre-wrap` collapses a text node's
+ * trailing newline and generates none, so the backdrop would come out exactly
+ * one line shorter and the mirrored offset would clamp a line above the caret.
+ */
+const DRAFT_TRAILING_NEWLINE = `${DRAFT}\n`
+
+/** The composer's two 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. */
+  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
+  /**
+   * 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.
+   */
+  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. */
+  backdropWrapWidth: number
+  /** Content width the hidden auto-grow mirror wraps at — it decides the box's height. */
+  mirrorWrapWidth: number
+}
+
+/**
+ * Measure both composer layers in the page.
+ * @param page - the page under test.
+ * @returns the two layers' offsets 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 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
+    // that decides the box's height, so its wrap width matters as much as the
+    // 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.
+    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 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
+    }
+    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
+    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),
+    }
+  }, { first: FIRST_MARKER, last: LAST_MARKER })
+}
+
+/**
+ * Render the golden body.
+ *
+ * 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.
+ * @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.
+ * @returns the golden body, without a trailing newline.
+ */
+function renderGeometry(top: ComposerMetrics, bottom: ComposerMetrics, trailingNewline: ComposerMetrics): string {
+  return [
+    '# Composer draft scrolling (14-line cap, two text layers)',
+    '',
+    '## 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)}`,
+    `- 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)}`,
+    `- 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)}`,
+    `- 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)}`,
+  ].join('\n').trimEnd()
+}
+
+describe('web e2e: composer draft scrolling', () => {
+  let scaffold: WebScaffold
+  let browser: Browser
+  let page: Page
+  let tripwire: ReturnType<typeof watchConsole>
+
+  beforeAll(async () => {
+    scaffold = await launchWebScaffold({})
+    browser = await chromium.launch()
+    page = await newEnglishPage(browser)
+    tripwire = watchConsole(page)
+    await page.goto(scaffold.baseUrl, { waitUntil: 'load' })
+    await page.waitForSelector('[class*="frame"]', { timeout: 30_000 })
+    await connectFreshWorkspace(page, 'composer-draft-scroll')
+    await page.locator('textarea:enabled').first().fill(DRAFT)
+  }, 180_000)
+
+  afterAll(async () => {
+    await browser?.close()
+    await scaffold?.close()
+  })
+
+  it('caps the draft box and keeps both text layers at the start', async () => {
+    onTestFailed(() => saveFailureShot(page, 'web-e2e-composer-draft-scroll-top'))
+    // Vacuity guard: without an overflowing draft there is nothing to scroll and
+    // every assertion below holds trivially.
+    await expect.poll(async () => (await measureComposer(page)).overflows, { timeout: 10_000 }).toBe(true)
+    // Typing the draft left the caret — and the box — at its end, so reach the
+    // start by the same gesture a user would, and leave it there for the wheel
+    // 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)
+    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)
+    expect(metrics.firstLineOffset).toBeGreaterThanOrEqual(0)
+    expect(metrics.firstLineOffset).toBeLessThan(metrics.clientHeight)
+    expect(metrics.lastLineOffset).toBeGreaterThan(metrics.clientHeight)
+    expect(tripwire.pageErrors).toEqual([])
+  }, 60_000)
+
+  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.
+    const metrics = await measureComposer(page)
+    expect(metrics.backdropWrapWidth).toBe(metrics.inputWrapWidth)
+    // The mirror decides the box height, so it belongs in the same equality —
+    // were it alone to wrap wider, the box would be measured too short and
+    // clip content before the 14-line cap, with every other assertion green.
+    expect(metrics.mirrorWrapWidth).toBe(metrics.inputWrapWidth)
+    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).
+    await page.mouse.wheel(0, 2000)
+    await expect.poll(async () => (await measureComposer(page)).inputScrollTop, { timeout: 10_000 })
+      .toBeGreaterThan(0)
+    const metrics = await measureComposer(page)
+    // The coupling, stated directly.
+    expect(metrics.layersAgree).toBe(true)
+    // 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.
+    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 () => {
+    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.
+    const input = page.locator('textarea:enabled').first()
+    await input.press('End')
+    await input.pressSequentially(' tail')
+    const metrics = await measureComposer(page)
+    expect(metrics.layersAgree).toBe(true)
+    expect(metrics.lastLineOffset).toBeGreaterThanOrEqual(0)
+    expect(metrics.lastLineOffset).toBeLessThan(metrics.clientHeight)
+    expect(tripwire.pageErrors).toEqual([])
+  }, 60_000)
+
+  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.
+    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
+    }, { 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)
+    expect(bottom.lastLineOffset).toBeGreaterThanOrEqual(0)
+    expect(bottom.lastLineOffset).toBeLessThan(bottom.clientHeight)
+    expect(tripwire.pageErrors).toEqual([])
+  }, 60_000)
+
+  it('matches the committed composer scroll geometry golden', async () => {
+    onTestFailed(() => saveFailureShot(page, 'web-e2e-composer-draft-scroll-golden'))
+    const input = page.locator('textarea:enabled').first()
+    // Restore the pristine draft (the edit case appended to it) and return to
+    // its start, both through ordinary gestures.
+    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)
+    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 })
+      .toBeGreaterThan(0)
+    const bottom = await measureComposer(page)
+    await input.fill(DRAFT_TRAILING_NEWLINE)
+    await input.hover()
+    await page.mouse.wheel(0, 4000)
+    await expect.poll(async () => {
+      const m = await measureComposer(page)
+      return m.inputScrollTop === m.inputMax
+    }, { timeout: 10_000 }).toBe(true)
+    const trailingNewline = await measureComposer(page)
+    await compareOrRefreshGolden(GEOMETRY_EXPECTED, renderGeometry(top, bottom, trailingNewline), MODE)
+    expect(tripwire.pageErrors).toEqual([])
+  }, 60_000)
+
+  it('commits exactly the fixtures it reads', async () => {
+    // Zero model calls, so the scenario records no session fixture: the geometry
+    // golden is the whole inventory.
+    await assertFixtureInventory(SNAPSHOT_DIR, ['geometry.expected.md'])
+  })
+
+  it.skipIf(MODE === 'record')('issued zero model calls and stayed clean', () => {
+    expect(tripwire.warnings).toEqual([])
+    expect(tripwire.pageErrors).toEqual([])
+  })
+})

apps/web/tests/snapshots/composer-draft-scroll/geometry.expected.md

@@ -0,0 +1,25 @@
+# Composer draft scrolling (14-line cap, two text layers)
+
+## At the start of the draft
+
+- draft overflows the capped box: true
+- visible lines: 14
+- both layers share one scroll extent: true
+- all three layers wrap at one width: true
+- textarea scroll offset: 0px
+- glyph layer tracks it: 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
+- 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

apps/web/tsconfig.json

@@ -40,6 +40,7 @@
     "tests/seeded-history.e2e.ts",
     "tests/sidebar-scrollbar.e2e.ts",
     "tests/code-mode-round.e2e.ts",
+    "tests/composer-draft-scroll.e2e.ts",
     "tests/cordis-tool-round.e2e.ts",
     "tests/message-actions.e2e.ts",
     "tests/queue-actions.e2e.ts",

packages/client/ui-conversation/src/client/skeleton/InputBar.module.css

@@ -193,6 +193,18 @@
   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. */
 }
 
 /* figma 34:10434: #ADB2B8 light / #81858C dark — the caption pair exactly. */

packages/client/ui-conversation/src/client/skeleton/InputBar.tsx

@@ -62,6 +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)
   // 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)
@@ -91,11 +92,22 @@ export function InputBar({
     if (!locked) inputRef.current?.focus()
   }, [locked, sessionId])
 
-  // Active conversation scrollport: chain the wheel. 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.
+  // 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.
   useEffect(() => {
     const el = inputRef.current
     if (el === null) return
@@ -108,8 +120,16 @@ 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 })
-    return () => { el.removeEventListener('wheel', onWheel) }
+    el.addEventListener('scroll', onScroll, { passive: true })
+    return () => {
+      el.removeEventListener('wheel', onWheel)
+      el.removeEventListener('scroll', onScroll)
+    }
   }, [])
 
   const onKeyDown = (e: KeyboardEvent<HTMLTextAreaElement>): void => {
@@ -355,6 +375,22 @@ 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 (
@@ -376,7 +412,7 @@ export function InputBar({
             (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 aria-hidden className={css.backdrop} data-input-backdrop>{backdrop}</div>
+          <div ref={backdropRef} aria-hidden className={css.backdrop} data-input-backdrop>{backdrop}</div>
           <textarea
             ref={inputRef}
             className={css.input}

packages/client/ui-conversation/tests/input-bar.spec.tsx

@@ -289,6 +289,37 @@ describe('running and lock semantics (queue cut 1)', () => {
     }
   })
 
+  it('the decoration backdrop tracks the textarea offset (it paints every visible glyph)', () => {
+    const { view, textarea } = bench({ draft: 'line\n'.repeat(40) })
+    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')
+  })
+
   it('disabled state shows the unavailable placeholder; custom placeholder wins', () => {
     const { textarea } = bench({ disabled: true })
     expect(textarea.placeholder).toBe('会话不可用')

tsconfig.host.json

@@ -27,6 +27,7 @@
     "apps/web/tests/seeded-history.e2e.ts",
     "apps/web/tests/sidebar-scrollbar.e2e.ts",
     "apps/web/tests/code-mode-round.e2e.ts",
+    "apps/web/tests/composer-draft-scroll.e2e.ts",
     "apps/web/tests/cordis-tool-round.e2e.ts",
     "apps/web/tests/message-actions.e2e.ts",
     "apps/web/tests/queue-actions.e2e.ts",