The renderer half. HUD mode reuses the app's own chat surface and only changes
the frame around it: no titlebar, no statusbar, no pane tree. The band and the
bar tile the window between them with no dead margins, and the band runs the
window's full height with the opaque bar sitting on its bottom edge, so there
is no seam to compute and none to drift as the composer grows.
Visibility is a WoW chat frame: the transcript shows while a turn is recent or
the composer has focus, then holds and fades. A bottom-anchored gradient mask
carries the fade on both the sheet and the text, and flattens away on focus so
nothing is dimmed while you are reading. Only the composer never fades — it is
the interface.
The window is mouse-transparent everywhere it isn't really there, so clicks over
the faded band reach the app behind it; `pointer-events` can't do this, since
the click never reaches the page at all.
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