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fix(ui-theme): gate the standard scrollbar properties behind the missing WebKit pseudo-element

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A non-`auto` `scrollbar-width` or `scrollbar-color` makes Chromium and Safari
discard every `::-webkit-scrollbar*` rule for that element, including
`::-webkit-scrollbar-thumb:hover`. Declaring both unconditionally left the
hover tokens rendering nowhere: the engines implementing the hover
pseudo-element are exactly the ones the standard properties silence, and
Firefox has no hover pseudo-element to fall back on. Both hover tokens and all
four elevated surfaces' hover rebinds were therefore dead code.

Measured in chromium on probe elements with `scrollbar-gutter: stable`: an 8px
`::-webkit-scrollbar` alone reserved a 30px band, and adding
`scrollbar-width: thin` dropped it to the 10px `thin` reserves.

The standard properties now sit inside `@supports not
selector(::-webkit-scrollbar)`, so Firefox takes them and WebKit-based engines
take the pseudo-elements. The WebKit rules stay ungated: an engine without
those pseudo-elements drops them as unknown selectors, and gating them would
hide them from an engine that implements them without `selector()` — the
pre-16.4 Safari the ungated form serves correctly.

Three unit assertions pin the split by source offset, which the existing
at-rule-flattening parser cannot see. The web e2e now reads the path chromium
actually takes: the `auto` standard properties as the gate's signature, the
pseudo-element sizing and track, the indirection variables resolved per
throwaway probe, and the hover declaration as cascade rule text — chromium
folds the `:hover` rule into `getComputedStyle(el,
'::-webkit-scrollbar-thumb')`, so no computed query separates the states.
This commit is contained in:
Chinesezjc
2026-07-28 15:02:20 +08:00
parent a6b0cd6f8d
commit c3e9690b2e
9 changed files with 251 additions and 80 deletions
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.agents/notes/implemented/bug-fix/2026-07-28-themed-scrollbars-and-reserved-gutter.i18n.yaml
+2 -2
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@@ -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/2026-07-28-themed-scrollbars-and-reserved-gutter.md
2026-07-28-themed-scrollbars-and-reserved-gutter.md: 404d1c037774aa24484cfac9227ad1d8b4d816d2
2026-07-28-themed-scrollbars-and-reserved-gutter.zh.md: ffad52e8dd0e72ca17eae058ee5cbf56764b22af
2026-07-28-themed-scrollbars-and-reserved-gutter.md: 71d2e5b5156d3bc54968552aa2eef82fab2ff443
2026-07-28-themed-scrollbars-and-reserved-gutter.zh.md: fcae03440ff621d949f8158990e2d871590b7daa