PR #27130 - manaflow-ai/cmux

Removes the extra background behind the trailing buttons in the Changes and wire Inspector headers, and behind a hovered tool output's Wrap and Copy buttons. Each button keeps its own hover and pressed fill. Tool output reserves space beside its text scroller so long lines remain readable with the controls visible.

Leo, verbatim:

"What the fuck is this? Why does this BACKGROUND STILL FUCKING EXIST ON THE RIGHT FUCKING BUTTONS??? ... How is this fucking issue been plaguing us for so fucking long?"

The transcript CSS fix is a separate commit so it can be split out independently. Risk: normal. No defaults change.

Verification

cd webviews && CMUX_BROWSER_TESTS=1 bun test test/browser/trailing-plates-diff.test.ts test/browser/trailing-plates-tool-output.test.ts

Before at 9ac87cc900f908a6488b0ec280d10771573b7006: four browser failures in light and dark: the header tools have a tinted background and inset shadow; the hovered output actions have a tinted background. After at 0227281a7173093365cea8fbd94c699b186eb534: all four pass, including visible Wrap/Copy, individual button hover, and text-scroller clearance in wrapped and unwrapped modes. The failing tests and fixture precede the two separate CSS fix commits.

Observed / Not run

Observed: Linux Chromium gallery captures in both themes, focused browser tests, and full web lint on Node 24. The output-action regression reveals controls through keyboard focus (:focus-within) for deterministic CI coverage, then checks button hover and both wrapped/unwrapped text clearance. The Changes and wire Inspector headers share the tested toolbar selector; the Inspector was inspected in source, not separately captured. Native UI capture was not run; this change is webview CSS only.