PR #35064 - manaflow-ai/cmux

Summary

Keep native window chrome geometry on live titlebar tokens and display pixels. Titlebar accessory reserves now derive from current traffic-light frames and backing scale, avoiding fractional seams and layout jolts as window chrome changes.

Leo's brief: smooth, purposeful window transitions and chrome geometry with no jolts or layout shift.

Performance

metric	before	after	delta	harness or run link
app compile check	pass	pass	0	job f669c21e7814c2fa8ea9dec5
titlebar geometry tests	existing coverage	expanded live-token/display-pixel coverage	+4 tests	`SidebarMinimumWidthTitlebarTests`

No frame-time claim is made; this is a geometry/token correction.

Observed / Not run

Observed: Linux app compile passed at 43953a03 (f669c21e7814c2fa8ea9dec5, 102 s).
Observed: focused titlebar geometry tests are included in the branch.
Not run: a new compile or capture; the coordinator's helper already ran the compile for this exact head.

Changelog

Changed: titlebar accessory geometry follows live titlebar tokens and device-pixel edges.

No capture: this geometry correction has no separate coordinator capture artifact; the existing nightly capture was blocked by system dialogs and missing non-DEV control socket.

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Summary by cubic

Keeps native window chrome geometry on live titlebar tokens and display pixels, so titlebar controls no longer land on fractional seams or jolt during token changes.

Titlebar accessory positioning now lives in WindowTitlebar and aligns to the window's backing scale.
The traffic-light reserve updates from live token changes instead of only window creation.
Adds pixel-grid alignment and live-token coverage to the titlebar geometry tests.