The overlay composer seat compensates for the bar instead of reserving a gutter
覆盖视图的 composer 座位改为补偿滚动条宽度，不再预留滚动条槽
The [composer-tab gutter reservation](2026-08-04-composer-tab-gutter-reservation.md) made the column's scroller reserve a scrollbar gutter unconditionally, so the composer seat measured the same width in Chat and in a view with a composer overlay. The cost was paid by every overlay view: the view's content column ended 8px short of the column's right edge, because the scroller reserved a gutter for a bar it never dra
English
Problem
The composer-tab gutter reservation made the column's scroller reserve a scrollbar gutter unconditionally, so the composer seat measured the same width in Chat and in a view with a composer overlay. The cost was paid by every overlay view: the view's content column ended 8px short of the column's right edge, because the scroller reserved a gutter for a bar it never draws — the trajectory ledger owns its own scrollers and the outer box never scrolls.
The trajectory table made that cost visible: its full-width row divider lines stopped 8px short of the pane edge, leaving a strip of whitespace at the right of every line and of the whole content column.
Decision
The reservation now belongs to Chat alone. The overlay branch declares scrollbar-gutter: auto, so the view's content spans the full column; the overlay composer seat (absolutely positioned against the padding box) gives back the bar's width with right: var(--dsh-scrollbar-width), so the input card still measures the same width as Chat's seat and does not move between tabs.
The compensation value is not a literal: ui-theme's scrollbar.css defines --dsh-scrollbar-width (8px on the WebKit path) beside the ::-webkit-scrollbar rule it mirrors, and the seat reads that variable. The scrollbar-styles spec pairs the variable with the mirrored rule and with the compensation consumer, so a change to the sheet's bar width without the variable — or to the variable without the consumer — fails the gate, not just review.
Alternatives considered
Keep the unconditional reservation and shrink every overlay view. The pre-fix behavior. It keeps one declaration for both tabs but taxes every overlay view with an 8px content column, which the trajectory ledger surfaced as visible whitespace. Rejected because the overlay views own their scrolling; they should not pay for Chat's bar.
Reserve on the overlay branch too and let the view bleed into the gutter. More moving parts for the same result: the gutter would still exist on a box that never scrolls, and the view would have to break out of the content box to reclaim its width.
Accept the 4px card shift. Dropping the reservation without compensating the seat would move the input card on every tab switch, which is exactly the symptom the earlier note fixed. Rejected: the card position is a deliberate cross-tab invariant.
Inset the overlay seat by the bar's width. The gutter-reservation note rejected exactly this, and this note adopts it; what changed is the rejection's premise. The number was the engine's, not ours — the WebKit path draws the sheet's 8px bar while the Firefox path draws whatever scrollbar-width: thin resolves to — so a hardcoded inset would line the two states up in Chromium and drift elsewhere. The overlay branch reserved an engine-resolved gutter of its own back then, so an inset had to match that width exactly. Today the overlay branch reserves nothing, so the compensation is the overlay side's only mechanism, and the literal half of the rejection is answered by making the 8px a variable that mirrors the ::-webkit-scrollbar rule in the same diff. The Firefox half remains: Chat reserves the engine-resolved width while the compensation stays fixed, and the residual drift where the two differ is recorded as an accepted cost in Consequences.
Consequences
- Chat keeps its reserved gutter and its stable card position; nothing changes on that tab.
- Overlay views (trajectory) span the full column; the trajectory ledger's divider lines reach the pane edge.
- The input card still holds one horizontal position across the Chat and Trajectory tabs, now by two mechanisms instead of one: Chat reserves, the overlay seat compensates.
- Chat reserves the engine-resolved width while the overlay seat compensates a fixed 8px. Where the two differ — the Firefox path resolves
scrollbar-width: thinper platform, and the e2e runs only on Chromium — the card drifts by half the difference on tab switch. Accepted residual cost, recorded here rather than asserted away: no measurement of the Firefox thin width on the target platforms exists in this change. --dsh-scrollbar-widthbecomes a public ui-theme variable read outside ui-theme; the scrollbar-styles spec pairs it with the mirrored::-webkit-scrollbarwidth rule and with the compensation consumer, closing the indirection-gate gap the variable would otherwise leave.
Testing
apps/web/tests/composer-tab-geometry.e2e.ts still asserts the card holds its position across tabs and now also asserts the split: Chat's scroller keeps scrollbar-gutter: stable and a nonzero band, while the overlay branch resolves auto with a zero band. The control cascade changed with the mechanism: it now drops the seat's right compensation (instead of dropping a gutter Chat never had on that branch) and measures the same 4px shift, proving the equal rectangles are not a tab switch that never reached layout. The committed golden records both states.
中文
问题
composer 标签页滚动条槽预留 让会话列滚动容器无条件预留一条滚动条槽，使 composer 座位在 Chat 与带 composer 覆盖的视图中测得相同宽度。代价由每个覆盖视图承担：视图内容列比列右边缘窄 8px，因为滚动容器为一条它从不绘制的滚动条预留了槽——trajectory 台账由视图内部自己的滚动容器滚动，外层盒子从不滚动。
trajectory 表格让这个代价显形：整行分隔线在面板右边缘前 8px 处停止，每条线右侧以及整个内容列右侧都留下一条空白带。
决策
预留现在只属于 Chat。覆盖分支声明 scrollbar-gutter: auto，视图内容占满整列；覆盖分支的 composer 座位（相对 padding box 绝对定位）用 right: var(--dsh-scrollbar-width) 让出滚动条宽度，使输入卡仍与 Chat 座位测得相同宽度，切换标签页时不移动。
补偿值不是字面量：ui-theme 的 scrollbar.css 在它镜像的 ::-webkit-scrollbar 规则旁定义 --dsh-scrollbar-width（WebKit 路径 8px），座位读取该变量。scrollbar-styles 规格把该变量与其镜像规则、以及补偿消费者配对检查，因此样式表滚动条宽度一变却不同步变量——或变量一变却不同步消费者——都会让门禁失败，而不只是评审时发现。
备选方案
保留无条件预留，压缩每个覆盖视图。 修复前行为。两个标签页一条声明，但每个覆盖视图都要付出 8px 内容列，trajectory 台账将其显现为可见空白。已拒绝：覆盖视图自己滚动，不应为 Chat 的滚动条买单。
覆盖分支也预留，并让视图渗入滚动条槽。 同样结果下更多活动部件：从不滚动的盒子上仍存在滚动条槽，视图还得突破内容盒才能取回宽度。
接受 4px 卡片位移。 去掉预留却不补偿座位，会在每次切换标签页时移动输入卡——正是前一份 note 修复的症状。已拒绝：卡片位置是刻意保持的跨标签页不变量。
把 overlay 座位按滚动条宽度内缩。 滚动条槽预留 note 当初否决的正是这个方案，本 note 采纳了它；变的是否决的前提。这个数字属于引擎而不属于我们——WebKit 路径绘制样式表里的 8px 滚动条，Firefox 路径绘制 scrollbar-width: thin 解析出的宽度——因此硬编码的内缩会让两种状态在 Chromium 上对齐、在别处继续漂移。当初 overlay 分支自己预留的是引擎解析出的槽宽，内缩必须精确匹配那个宽度。如今 overlay 分支不预留任何槽位，补偿成为覆盖侧唯一的机制；否决的字面量那一半，通过把 8px 变成与 ::-webkit-scrollbar 规则同处一个 diff 的变量来回应。Firefox 那一半仍然存在：Chat 预留引擎解析宽度，补偿保持固定 8px，两者不等之处的残余漂移作为接受的代价记录在后果中。
后果
- Chat 保留滚动条槽与稳定的卡片位置；该标签页无任何变化。
- 覆盖视图（trajectory）占满整列；trajectory 台账的分隔线到达面板右边缘。
- 输入卡在 Chat 与 Trajectory 标签页间仍保持同一水平位置，现在由两种机制而非一种达成：Chat 预留，覆盖座位补偿。
- Chat 预留引擎解析宽度，覆盖座位补偿固定的 8px。两者不等之处——Firefox 路径按平台解析
scrollbar-width: thin，而 e2e 只在 Chromium 上运行——卡片在切换标签页时会漂移半个差值。这是接受的残余代价，如实记录于此而不断言消除：本次改动并未提供目标平台 Firefox thin 宽度的实测。 --dsh-scrollbar-width成为 ui-theme 对外、且被 ui-theme 之外读取的变量；scrollbar-styles 规格把它与镜像的::-webkit-scrollbar宽度规则、以及补偿消费者配对检查，补上了该变量本会留下的间接层门禁缺口。
测试
apps/web/tests/composer-tab-geometry.e2e.ts 仍断言输入卡在标签页间保持位置，并新增断言拆分：Chat 滚动容器保持 scrollbar-gutter: stable 与非零槽宽，覆盖分支解析为 auto 且槽宽为零。控制级联随机制改变：现在移除座位的 right 补偿（而非移除该分支上 Chat 从未有过的槽），测得同样的 4px 位移，证明相等的矩形并非从未到达布局的标签页切换。提交的 golden 记录两种状态。