# Agent Note: The scrollbar tokens get their consumer, and the workspace list reserves its gutter

Status: implemented

English | [中文](2026-07-28-themed-scrollbars-and-reserved-gutter.zh.md)

## Problem

`design-platform.css` declares four `--dsw-alias-scrollbar-*` tokens (`bg-l1`, `bg-l2`, `hover-l1`, `hover-l2`) in both palettes, and no rule anywhere in the client read them. A defined token with no consumer is not a theme: every scrolling region rendered the user agent's own scrollbar, which knows nothing about the palette, so the dark theme showed a light native bar against dark surfaces.

The visible symptom that surfaced the gap was elsewhere. The workspace browser's session list (`.list` in `WorkspaceBrowser.module.css`) is the sidebar's only scrolling region, and each row's trailing content sits flush against the row's 8px right padding — `.time` in `rows/Rows.module.css` is `flex: none`, as are the action buttons that replace it on hover. An overlaid scrollbar therefore painted on top of the relative timestamp. Reserving space in that one list would have left the bar itself unthemed, so the two halves are one change.

## Decision

`packages/client/ui-theme/src/styles/scrollbar.css` is the sole consumer of the four tokens, and the fifth ui-theme sheet in the shell's import chain (`packages/client/web/src/base.css`). It follows `design-platform.css` there because it reads that sheet's tokens.

The rules sit on `body`, not `html`. `design-platform.css` declares the `--dsw-alias-*` tokens on `body`, with the dark overrides on `body[data-ds-dark-theme]`, and custom properties inherit only downward; an `html` rule resolves them to the guaranteed-invalid value, at which point `scrollbar-color` computes to `auto` and no theming happens at all.

`scrollbar-width` and `scrollbar-color` are declared on `body, body *` rather than once at the top. Inheritance would pass down the color already substituted at `body`, so a descendant rebinding the indirection could not change its own scrollbar; re-declaring makes each element substitute the variable as it sees it. `scrollbar-width` is not an inherited property in the first place, so it needs the per-element declaration regardless. The `::-webkit-scrollbar*` pseudo-elements are likewise not inherited and are matched unscoped.

The two renderings are mutually exclusive, and the exclusion is enforced rather than assumed. A non-`auto` `scrollbar-width` or `scrollbar-color` makes Chromium and Safari discard every `::-webkit-scrollbar*` rule for that element, `::-webkit-scrollbar-thumb:hover` included. Declaring both unconditionally therefore leaves the hover token rendering nowhere at all: the engines that implement the hover pseudo-element are exactly the ones the standard properties silence, and Firefox has no hover pseudo-element to fall back on. The standard properties consequently sit inside `@supports not selector(::-webkit-scrollbar)`, which is true only where the pseudo-element is unimplemented, so Firefox takes the standard path and WebKit-based engines take the pseudo-element path. The WebKit rules are not gated in turn: an engine without those pseudo-elements drops them as unknown selectors, so a gate would only restate what selector matching already does. An engine too old for the `selector()` function makes the condition invalid, which evaluates false and selects the pseudo-element path — the correct side for the pre-16.4 Safari that is the realistic case for that reading.

Both paths read one indirection pair, `--dsh-scrollbar-thumb` and `--dsh-scrollbar-thumb-hover`, bound on `body` to the l1 (base-surface) tokens. **This is the rebinding contract, and it is the part the CSS alone does not state**: an elevated surface sets `--dsh-scrollbar-thumb: var(--dsw-alias-scrollbar-bg-l2)` and `--dsh-scrollbar-thumb-hover: var(--dsw-alias-scrollbar-hover-l2)` on its own container, and that one rebind reaches the standard properties and the WebKit pseudo-elements together. The pair is rebound as a pair; rebinding the resting thumb alone leaves the hover state on the base-surface token. `transparent` is the pair's other legal target, added when the sidebar's bars [started following the pointer](../feature/2026-08-04-pointer-revealed-sidebar-scrollbars.md); the gate below admits those two and nothing else. The mechanically discoverable subset is owned by `packages/client/ui-theme/tests/scrollbar-styles.spec.ts`: any sheet that both scrolls and paints an elevated surface must rebind, so this note no longer maintains a complete surface inventory. Most declare the pair on the elevated card rather than on the scrolling descendant, because elevation belongs to the surface and custom properties inherit to whichever child actually scrolls.

Four surfaces — `Menu`, `InputBar`, `QuestionComposer`, and `TodoPanel` — were missed in the first implementation and found in review, which is why the per-sheet rebinding contract is checked mechanically rather than by inspection.

The elevated set is resolved from the palette's own dark elevation ladder — the surface tokens whose dark value lands on `bg-layer-2` or `bg-layer-3`, which is the step the l1/l2 split encodes. Deriving it instead from the sheets that already rebind was the first attempt and is unsound: such a set can only confirm what someone already remembered, and a surface nobody has rebound yet — exactly the case the check exists for — defines itself as unelevated. `--dsw-specific-tip` proved it, resolving to the menu surface's rung while the todo panel scrolled on it unrebound and the derived check stayed green.

Scope is by token family, not by geometry: only `--dsw-alias-bg-*` and `--dsw-specific-*` name a surface. `--dsw-alias-button-*`, `--dsw-alias-interactive-*`, and `--dsw-alias-markdown-*` reach the same rungs while naming a control or an inline span that no scroll container renders its bar against. Shape cannot make that call, since a floating button legitimately carries a radius, a shadow, and a fixed size. The check is per sheet rather than per rule because the card and the descendant that scrolls are separate rules. That approximation cannot detect a scrolling component embedded in an elevated card painted by another package's stylesheet, as `DirectoryBrowser` inside `Modal` demonstrated; cross-sheet composition remains a review and assembled-UI responsibility.

The track and the corner stay transparent, so the thumb reads against whatever surface scrolls under it; only the thumb and its hover state carry a token color.

`.list` declares `scrollbar-gutter: stable`, which keeps the bar beside the rows instead of on top of them. `stable` rather than `auto` because `auto` reserves the gutter only while the list actually overflows: expanding a workspace group would then shift every row horizontally at the moment it starts scrolling. The reservation is unconditional and the rows never move.

The gutter and the sheet's `::-webkit-scrollbar` width are jointly necessary against an overlay scrollbar, which is the configuration where the symptom exists at all. Measured on the running app by deleting each from the live cascade with the other left in force: either deletion alone takes the list's band from 8 to 0. The gutter states that space be reserved, and the pseudo-element width is what makes chromium treat the bar as occupying layout space rather than floating over the content. Neither half of this change is therefore optional for the reported bug, which is a second reason the two halves ship together.

## Alternatives considered

**Per-module `::-webkit-scrollbar` rules in each scrolling component sheet.** Rejected: the client has thirteen scrolling containers across nine packages, every one would carry the same block, and the fourteenth would ship unthemed with nothing failing. A skin driven by design tokens belongs in the package that owns the tokens.

**An opt-in utility class that each scroll container adds.** Same duplication removed, but the failure mode stays: a new scroll container is themed only if its author remembers the class, and the omission is invisible in review. The `body, body *` form has no opt-in step to forget; a container that genuinely wants a different bar overrides the indirection, which is the same mechanism elevated surfaces use.

**Bind the properties on `html`.** The natural place for a document-wide skin, and it fails measurably: with the rule on `html` a scroll container computes `scrollbar-color: auto` in chromium, because the alias tokens are not in scope there.

**Declare the properties once and let them inherit.** Fewer matched elements, and it breaks the rebinding contract — inheritance carries the substituted color, not the variable reference, so an elevated surface could not retint its own scrollbar. It is also incomplete on its own terms, since `scrollbar-width` does not inherit.

**Declare the standard properties and the pseudo-elements unconditionally, without the `@supports` gate.** This is what the change originally shipped, and review caught it. Measured in chromium on probe elements with `scrollbar-gutter: stable` so the band is observable: an 8px `::-webkit-scrollbar` alone reserved a 30px band (the sheet's width plus the UA's buttons), and adding `scrollbar-width: thin` to the same element dropped it to the 10px `thin` reserves — the pseudo-element rules were being discarded, not merged. Every `::-webkit-scrollbar-thumb:hover` rule went with them, so both hover tokens and all four elevated surfaces' hover rebinds were dead code on the engine most users run.

**Gate the WebKit rules too, behind `@supports selector(::-webkit-scrollbar)`.** Symmetrical to read, and wrong in one direction: it would hide the rules from an engine that implements the pseudo-elements but not `selector()`, which is the pre-16.4 Safari the ungated form serves correctly. Unknown selectors are already dropped, so the gate adds no protection to pay for that.

**Pad the rows instead of reserving the gutter (extra right padding on `.list`, or moving `.time` inward).** Rejected: padding applies whether or not a bar is present, so it costs horizontal room in the common short-list case, and it fixes exactly one container while leaving every other scrolling region's content under its bar.

**`scrollbar-gutter: auto` on `.list`.** The reservation appears when the list overflows, which is when the bar exists. Rejected because the sidebar's lists grow and shrink as groups expand, so the reservation would appear and disappear under the user's cursor and shift the rows with it.

## Consequences

- Every scroll container in the client draws the themed thumb: `rgb(229, 229, 229)` on a light base surface, `rgb(60, 60, 61)` on a dark one, and `rgb(84, 85, 87)` for a dark elevated surface that rebinds to the l2 pair. The sidebar's regions draw theirs only under the pointer, through the same indirection.
- The two renderings are separately specified, so a change to the thumb's geometry or hover behavior has to be made twice — once in `scrollbar-width`/`scrollbar-color`, once in the pseudo-elements. Routing both through the indirection pair confines that duplication to the properties Firefox and WebKit do not share.
- The hover tokens (`--dsw-alias-scrollbar-hover-l1`/`-l2`) render only on the pseudo-element path. Firefox states one thumb color through `scrollbar-color` and derives its own hover treatment, so a design change to the hover colors is visible in Chromium and Safari and not in Firefox. This is a limit of `scrollbar-color`, not of the sheet.
- `body *` matches every element, for two properties whose effect the user agent already limits to elements that actually scroll. The cost is a broad selector; the alternative was a rebinding contract that does not work.
- The workspace list is permanently narrower by the reserved band, at every list length. That is the trade the fix buys: stable row geometry instead of a timestamp that is legible only while the list is short.
- There is no track token in the palette, so a design that later wants an opaque track needs a new alias token rather than a literal color in this sheet.

## Testing

Three unit specs read the CSS text on disk. `ui-theme/tests/scrollbar-styles.spec.ts` scans the scrollbar token set out of `design-platform.css` rather than hardcoding it, so adding, renaming, or dropping a token moves the assertions with it, and checks that every token has a consumer and that each elevated surface rebinds a complete pair. It also pins the path split by source offset: the standard properties inside the gate block, the `::-webkit-scrollbar*` rules and every read of the hover indirection outside it. That split needs an offset assertion because the spec's rule parser flattens through at-rules, so a gate deleted or a declaration moved across it leaves every other assertion in the file green.

`apps/web/tests/sidebar-scrollbar.e2e.ts` covers the facts only a real engine reports: the reserved band width, and which rendering path the engine took. It needs no model calls — the list only has to overflow — so it seeds cold sessions from an existing committed fixture read-only.

That scenario also commits a golden, `snapshots/sidebar-scrollbar/geometry.expected.md`, holding the resolved scrollbar style and geometry in both palettes. The aria goldens the other web scenarios commit cannot carry a CSS-only change: it alters no DOM and no accessible name, so their normalized trees are byte-identical with and without it. Recording the resolved values instead makes an unintended shift in thumb colour, band width, or rendering path a reviewable diff rather than a threshold someone has to reason about. Absolute coordinates are deliberately excluded — `timeRight` and the two edges depend on the sidebar's laid-out width and on font metrics, so committing them would produce a fixture that has to be re-recorded per platform and would document the platform rather than the change. What is recorded is the band, the overlap, and two orderings, each a difference or a comparison that survives any layout preserving the reservation.

Confirmed in headless chromium on the built client by reading computed values, which is what distinguishes a working token chain from a syntactically valid one: a scroll container computes the l1 thumb color in each palette, and a container that rebinds the indirection computes the l2 color, proving the rebind reaches the computed value rather than only the custom property. Firefox was verified the same way for the standard path, including the l1-to-l2 rebind on `scrollbar-color`; headless Firefox reports `scrollbar-width: none` on every element, styled or not, which is a headless artifact rather than an effect of the sheet.

Two chromium measurement limits shape what the e2e can assert. The gate makes chromium report `scrollbar-width` and `scrollbar-color` as `auto`, so the substituted `scrollbar-color` is no longer the observable — the e2e asserts the `auto` reading deliberately, since a concrete value there would mean the gate leaked and silenced the pseudo-elements. And `getComputedStyle(el, '::-webkit-scrollbar-thumb')` folds in the `::-webkit-scrollbar-thumb:hover` rule, so it reports the hover color at rest and pins neither state; proven by deleting the hover rule through `CSSStyleSheet.deleteRule` in the live page, which flipped that same query from the hover color to the resting one. The e2e therefore reads the resting and hover colors as the indirection variables resolve on the list — one throwaway probe element per variable, because `getComputedStyle` returns a live declaration and a reused probe reports only the last value read — and reads the hover declaration out of the cascade as rule text.

The gate itself has a negative control at the level it operates on: removing the `@supports` wrapper from the sheet, rebuilding `build:web`, and rerunning the e2e turns the `scrollbar-width: auto` assertion red with `thin`, which is the suppression the gate exists to prevent.

Headless chromium draws overlay scrollbars, and that is the configuration in which the reported symptom exists, so the e2e reproduces the bug rather than approximating it: against clean master the list's band is 0 and the bar covers 7px of the relative time. A reserved gutter there does not shrink `clientWidth`, so an assertion comparing the time element's right edge against the client-area edge holds with and without the reservation and would pass or fail on the platform's scrollbar style rather than on the declaration under test. The two signals that do separate the states are the `offsetWidth - clientWidth` band and `timeCoveredBy`, the overlap measured against the bar's own width.

Both are asserted because each catches a different regression, established by mutating one declaration at a time with the other assertions in that test silenced. Removing only the gutter leaves `timeCoveredBy` at 0 — the bar is then 8px and the row's right padding is also 8px, so it abuts the timestamp without covering it — and the band assertion is what fails. Removing the pseudo-element width as well, which is the actual master state, produces the overlap, and `timeCoveredBy` fails at 7. A headed run under xvfb cannot show the symptom in either state, because chromium paints a classic space-consuming bar there and `clientWidth` already excludes it.

Verifying browser-visible plugin CSS needs a rebuild `pnpm run build:web` does not perform. `WorkspaceBrowser.module.css` never reaches `apps/web/dist`: ui-workspace loads as a runtime plugin and its CSS is inlined into `packages/client/ui-workspace/lib/client.js`, built by that package's own `bundle` script. A negative control that reruns only `build:web` therefore exercises a stale bundle and passes with the declaration removed, which reads as a vacuous test rather than as an invalid control. Rebuild with `pnpm --filter @deepseek-ai/dsh-client-ui-workspace run bundle`, confirm the artifact by grepping `lib/client.js` for the declaration, then `build:web`.

`test:web` ran `build:web` alone, so every scroll-region or plugin-CSS change hit that trap; it now runs `build` first, which covers `packages/*/*` and so rebuilds the plugin bundles. `check-all` already ordered `build` before `build:web`, so CI was never exposed — only the local script was, which is exactly where a stale-bundle pass is most likely to be believed.
