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c3e9690b2e
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.