docs(web): cite the surviving composer scrollport note for the WebKit gutter measurement

.agents/notes/implemented/bug-fix/2026-08-04-composer-tab-gutter-reservation.i18n.yaml

@@ -2,5 +2,5 @@
 # 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/2026-08-04-composer-tab-gutter-reservation.md
-2026-08-04-composer-tab-gutter-reservation.md: f9a0c346c0c023fe2602486376b9794781f824db
-2026-08-04-composer-tab-gutter-reservation.zh.md: 67a284ec0a12e464ad6f1dc17e954bdebd28e80d
+2026-08-04-composer-tab-gutter-reservation.md: 3b28c35c1f11676e41cabde76d1b0d16c688f034
+2026-08-04-composer-tab-gutter-reservation.zh.md: 26e8b6bff73a01e6518f3918d201330c1d029876

.agents/notes/implemented/bug-fix/2026-08-04-composer-tab-gutter-reservation.md

@@ -18,7 +18,7 @@ So for as long as the transcript overflowed — the ordinary state of any sessio
 
 The two halves are one change. The reservation is what makes both states measure against the same width; declaring the overlay branch a scroll container is what makes the reservation reach it. `stable` rather than `auto` because `auto` reserves only while the box actually overflows, and the difference between overflowing and not is precisely the difference between the two tabs — an `auto` gutter would state the bug rather than fix it.
 
-The overlay state is a scroll container that nothing scrolls: the view fills it (`flex: 1 1 0` with its own clip) and the seat is out of flow, so no gesture and no clipping behavior changes. What changes is which declarations the engine honours. WebKit applies `scrollbar-gutter` to an `overflow-y: auto` box and ignores it on a hidden one — measured on this app's own composer layers and recorded in [the composer glyph-layer note](2026-07-31-composer-glyph-layer-tracks-the-textarea.md) — so a reservation left on a hidden box would hold in Chromium and silently not in Safari.
+The overlay state is a scroll container that nothing scrolls: the view fills it (`flex: 1 1 0` with its own clip) and the seat is out of flow, so no gesture and no clipping behavior changes. What changes is which declarations the engine honours. WebKit applies `scrollbar-gutter` to an `overflow-y: auto` box and ignores it on a hidden one — measured on this app's own composer layers and recorded in [the composer scrollport note](2026-07-31-composer-text-layers-share-one-scrollport.md) — so a reservation left on a hidden box would hold in Chromium and silently not in Safari.
 
 The horizontal axis is declared rather than left to compute: a box that scrolls on one axis computes `visible` on the other to `auto`, and would grow a horizontal scrollbar of its own the first time a view's content reached past the column.
 

.agents/notes/implemented/bug-fix/2026-08-04-composer-tab-gutter-reservation.zh.md

@@ -18,7 +18,7 @@ composer 座位在组件树中只有一个节点、一个位置，但它究竟
 
 这两半是同一处改动。预留使两种状态依附于同一个宽度；把 overlay 分支声明为滚动容器，才使这条预留真正抵达它。选 `stable` 而非 `auto`，是因为 `auto` 只在盒子确实溢出时才预留，而"溢出与否"恰恰就是两个标签页之间的那点差别——`auto` 的写法只是把缺陷重述一遍，并不能修掉它。
 
-overlay 状态是一个没有任何东西会去滚动它的滚动容器：视图把它填满（`flex: 1 1 0`，且自带裁剪），座位不在常规流中，因此没有任何手势与裁剪行为发生变化。变化的是引擎会认哪些声明。WebKit 对 `overflow-y: auto` 的盒子应用 `scrollbar-gutter`，对 hidden 的盒子则忽略它——这是在本应用 composer 自身的图层上实测所得，并记录于 [composer 字形层记录](2026-07-31-composer-glyph-layer-tracks-the-textarea.md)——所以把预留留在一个 hidden 盒子上，会在 Chromium 上成立，在 Safari 上悄无声息地不成立。
+overlay 状态是一个没有任何东西会去滚动它的滚动容器：视图把它填满（`flex: 1 1 0`，且自带裁剪），座位不在常规流中，因此没有任何手势与裁剪行为发生变化。变化的是引擎会认哪些声明。WebKit 对 `overflow-y: auto` 的盒子应用 `scrollbar-gutter`，对 hidden 的盒子则忽略它——这是在本应用 composer 自身的图层上实测所得，并记录于 [composer 滚动容器记录](2026-07-31-composer-text-layers-share-one-scrollport.md)——所以把预留留在一个 hidden 盒子上，会在 Chromium 上成立，在 Safari 上悄无声息地不成立。
 
 横向轴是显式声明的，而不是交给推导：单轴滚动的盒子会把另一轴的 `visible` 计算为 `auto`，于是只要某个视图的内容第一次伸出列外，它就会长出自己的横向滚动条。
 

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

@@ -227,8 +227,9 @@
      of flow — but declared as a scroll container on the same axes as the Chat
      state instead of `overflow: hidden`, so the reservation above reaches this
      state too: WebKit honours `scrollbar-gutter` on an `overflow-y: auto` box
-     and ignores it on a hidden one (measured for the composer's own layers,
-     see InputBar.module.css). The horizontal axis is declared rather than left
+     and ignores it on a hidden one (measured for the composer's own layers —
+     .agents/notes/implemented/bug-fix/2026-08-04-composer-tab-gutter-reservation.md
+     cites the measurement). The horizontal axis is declared rather than left
      to compute, because a box that scrolls on one axis computes `visible` on
      the other to `auto` and would grow a horizontal bar of its own. */
   overflow-x: hidden;