Fix Chromium agent chat tab-row tools background

Closed
TE
3d ago
+79-32

HOLD: CEF before/after has not reproduced the reported box. Verify the explicit agent surface override and preserve its fill below the row before landing.

Chromium agent chats can leave an opaque document surface behind the top-right tools even when the tab-row shell is transparent. Leo: “Why is the button area still, like, just garbage? What is this? There's still that line of that box with the background that just always is there”.

The shipped agent-pane bundle lacks the top-level document transparency rule. Add the scoped html:has(.acpmux-shell[data-tab-row]) rule to the stylesheet that ships in the pane, overriding WebTheme's document-start background. The shell paints the shared surface below that row so explicit chat color and opacity settings remain intact. Tools stay in the tab row with their existing control-sized hover/press states. Native holes continue to expose the tabs and trailing toolbar.

Validation:

  • Chromium regression fixture: models the opaque !important bootstrap, asserts zero pixel alpha across the offered row, and checks legacy, opaque, translucent, and transparent fills below (28 tests passed). Browser suite passed.
  • Fleet AgentPaneTabRowHolesTests: 4/4 at the corrected Swift tree, 3/3 on merge base; no crashes. The added test protects the tools slice from native occlusion; it does not prove compositor transparency.
  • Fast gates passed at the preceding head. Independent Astra review passed at e78f39c50a1a1ff6f610853da291e499f42c48a3; final-head fast gates are pending for the surface-override follow-up.

Before/after CEF captures: real Chromium was confirmed at both merge base and head. Light and dark stills show no visible difference, and neither reproduces the reported box or seam. These are Chromium no-regression evidence; the browser fixture supplies regression coverage for the opaque-root defect. Screenshot upload is currently blocked by the media publishing path; the captured stills will be linked when publishing is repaired. The original visual complaint remains unconfirmed in the fleet capture.

Risk: this clears the document root only while a chat shares the tab row; the pane paints the shared surface below the row, falling back to its legacy page color when that token is absent. CEF confirmation with an explicit surface override is pending.

Agent session: agent session