The HUD asked for vibrancy directly and always with the 'hud' material —
one of the two rungs the macOS census rejected, because it collapses into
under-window on blur and so changed the frost the moment another app took
focus. It also ignored the translucency setting entirely: Glass off still
frosted, and Windows got nothing at all.
hudFrostFor is the mapping for a transparent window, beside vibrancyFor in
the shared module both processes read. Two gates give it its answer: the
renderer's report that the band actually covers the window, and the user's
Glass setting. Off resolves to no material rather than a resting one, since
a transparent window has no opaque page to hide an unwanted frost behind.
Windows 11 rides setBackgroundMaterial through the same call, so the HUD
follows the frost ladder on both platforms. Main self-diffs and keys the
latch to the window, so a Settings change re-frosts a live HUD, a tint drag
touches nothing native, and a HUD respawned on another profile is not
mistaken for the window that already carried the material.
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