feat: optimize chat page scroll area

.agents/notes/implemented/bug-fix/2026-07-29-sticky-composer-conversation-scroll.i18n.yaml

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+# 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/2026-07-29-sticky-composer-conversation-scroll.md
+2026-07-29-sticky-composer-conversation-scroll.md: f245e7ca0404df4f644504ac9e1b101e659b6b68
+2026-07-29-sticky-composer-conversation-scroll.zh.md: 7e4d5fb523f33f450c343e12e486e1d33784ee6c

.agents/notes/implemented/bug-fix/2026-07-29-sticky-composer-conversation-scroll.md

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+# Agent Note: Fixed header, sticky composer inside the transcript scrollport
+
+Status: implemented
+
+English | [中文](2026-07-29-sticky-composer-conversation-scroll.zh.md)
+
+## Problem
+
+The active conversation column split scrolling: the chat (and trajectory) view owned `overflow-y: auto`, while the composer stack sat as a sibling below that scrollport. A wheel gesture over the stats line or input therefore hit a non-scrolling region and did nothing — the transcript only moved when the pointer was over the message list. Long drafts made it worse: the textarea is itself a scrollport, so wheel over the composer could be trapped there. The session header must occupy the top of the column as ordinary chrome (not `position: sticky` inside the scrollport), while the composer must stick to the bottom of the same scrollport as the transcript so wheel over the footer moves the flow.
+
+## Decision
+
+Active phase keeps the session header as `flex: none` column chrome above the scrollport. `ConversationRoot` supplies a `wrapActiveBody` owner callback that wraps the view ring in a `data-conversation-scroll` body and places the composer stack inside that body with `position: sticky; bottom: 0`. Hero and settling keep the composer as a Root child (centered hero card). ChatView and Trajectory/Waterfall keep a local scroller only when mounted outside that host (unit tests); under the host they set `overflow: visible` and resolve bottom-follow / prepend anchoring through `closest('[data-conversation-scroll]')`.
+
+Session stats live on `'conversation.composer.dock'` (above `'conversation.input.dock'`). The InputBar textarea, when inside the host, listens for `wheel` with `{ passive: false }`, calls `preventDefault`, and applies `deltaY` to the host — hero mounts have no host and keep native textarea wheel behavior. Moving the composer into the Session scrollport on the hero → active flip may remount the textarea; the InputHub draft is the durable carrier across that flip.
+
+## Alternatives considered
+
+**Sticky header and sticky composer inside one column scrollport.** Rejected for the header: it must occupy the top as fixed layout chrome, not participate in the scrollport's sticky layer.
+
+**Fixed flex-none composer below the scrollport with wheel forwarding.** Rejected: the product requires the composer to stick inside the transcript scrollport so the footer is part of that scroll hit-testing surface, not a sibling that only forwards deltas.
+
+**Portal the composer into ChatView's scroller.** Rejected: the composer is shared across view tabs; the wrap target is the Session body owned by the resident shell.
+
+**Keep StatsLine inside ChatView below the message column.** Rejected: outside the sticky composer it would scroll away while the input stayed pinned.
+
+## Consequences
+
+Wheel over the footer scrolls the transcript; the visible layout is a fixed header, scrolling transcript, and sticky bottom composer. Stats appear on every active view tab. Nested view scrollers under the host are suppressed so sticky Turn headers in Trajectory stick to the column host. Hero → active asserts draft survival through the InputHub, not textarea DOM identity.

.agents/notes/implemented/bug-fix/2026-07-29-sticky-composer-conversation-scroll.zh.md

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+# Agent Note: 固定标题栏，sticky 编辑器位于 transcript 滚动容器内
+
+Status: implemented
+
+[English](2026-07-29-sticky-composer-conversation-scroll.md) | 中文
+
+## Problem
+
+活跃会话列把滚动拆成两段：聊天（以及 trajectory）视图自有 `overflow-y: auto`，编辑器栈则作为该滚动容器的兄弟节点坐在下方。指针落在统计行或输入区上时，滚轮打在不可滚动区域上因而毫无效果——只有指针在消息列表上时 transcript 才会移动。草稿变长时更糟：textarea 本身也是滚动容器，编辑器上的滚轮可能被截在那里。会话标题栏必须以普通 chrome 占据列顶（不能在滚动容器内 `position: sticky`），而编辑器必须与 transcript 贴在同一滚动容器底部，使页脚上的滚轮能带动内容流动。
+
+## Decision
+
+活跃阶段会话标题栏保持为滚动容器之上的 `flex: none` 列 chrome。`ConversationRoot` 提供 `wrapActiveBody` owner 回调，将视图环包进 `data-conversation-scroll` 主体，并把编辑器栈以 `position: sticky; bottom: 0` 放进该主体。Hero／settling 仍把编辑器作为 Root 子节点（居中 hero 卡片）。ChatView 与 Trajectory/Waterfall 仅在宿主之外挂载时（单元测试）保留本地 scroller；位于宿主下时设为 `overflow: visible`，并通过 `closest('[data-conversation-scroll]')` 解析贴底跟随与前置锚定。
+
+会话统计挂在 `'conversation.composer.dock'`（位于 `'conversation.input.dock'` 之上）。InputBar 的 textarea 在宿主内以 `{ passive: false }` 监听 `wheel`，调用 `preventDefault`，并将 `deltaY` 施加到宿主——hero 挂载没有宿主，保留 textarea 原生滚轮行为。hero → active 翻转时编辑器进入 Session 滚动容器可能重挂载 textarea；跨该翻转的耐久载体是 InputHub 草稿。
+
+## Alternatives considered
+
+**标题栏与编辑器都在同一列滚动容器内 sticky。** 标题栏否决：它必须作为固定布局 chrome 占据顶部，而不是参与滚动容器的 sticky 层。
+
+**滚动容器下方 flex-none 固定编辑器并转发滚轮。** 否决：产品要求编辑器 sticky 在 transcript 滚动容器内，使页脚成为该滚动命中面的一部分，而不是仅转发增量的兄弟节点。
+
+**把编辑器 portal 进 ChatView 的 scroller。** 否决：编辑器跨视图标签共享；包装目标是常驻壳拥有的 Session 主体。
+
+**把 StatsLine 留在 ChatView 消息列下方。** 否决：落在 sticky 编辑器之外会随内容滚走，而输入区仍钉在底部。
+
+## Consequences
+
+在页脚上滚轮会滚动 transcript；可见布局是固定标题栏、可滚动 transcript 与 sticky 底部编辑器。统计出现在每一个活跃视图标签上。宿主下的嵌套视图 scroller 被抑制，因而 Trajectory 的 sticky Turn 标题贴在列宿主上。hero → active 断言经 InputHub 的草稿存续，而非 textarea DOM 身份。