Now that the popped-out composer is fixed to the viewport, clamping against the
window let it slide under a pinned sidebar. Confine it to the thread region
(data-slot="composer-bounds") instead — its rect already excludes a pinned
sidebar and the header — falling back to the full window before it's measured.
This subsumes the old titlebar top-margin (the thread rect starts below the
header).
Brooklyn Nicholson
ea5fa505d9