# Agent Note: The overlay composer seat compensates for the bar instead of reserving a gutter

Status: implemented

English | [中文](2026-08-12-composer-overlay-seat-width-compensation.zh.md)

## Problem

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 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](2026-08-04-composer-tab-gutter-reservation.md) 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 rejected design assumed an engine-resolved gutter in the overlay branch, so an inset had to match that width exactly. 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: thin` per 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-width` becomes a public ui-theme variable read outside ui-theme; the scrollbar-styles spec pairs it with the mirrored `::-webkit-scrollbar` width 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.
