The frost is native vibrancy, which is the window's content view rather than
an element — it fills the whole rectangle and nothing in the page can clip it
to the sheet. That was only ever right while the sheet covered the window;
anywhere it falls short, the difference is frost over empty space. On a fresh
thread the sheet is zero and the difference is the entire window, which is the
grey slab that appears the moment you put the caret in the composer.
Gating the caller's `engaged` was not enough, and is why the first attempt at
this missed: the hook turns the frost on for a focused composer by itself,
independent of what the caller passes. The veto belongs inside, next to that
check.
Brooklyn Nicholson
f03fc46845