useComposerMetrics dedupes its --composer-measured-height writes against
a "last published bucket" ref. The unmount cleanup cleared the surface
vars but left that ref alone, so after a non-final unmount (StrictMode
replay, Suspense hide) the re-mount measured the same dock, saw an
unchanged bucket, and never wrote the var back. The thread then read
the :root estimate (~62px) under a dock that could be 200px tall, and
the status stack covered the last turn until a real resize fired.
Reset the bucket refs in the same cleanup that clears the vars. Adds a
StrictMode regression test that fails on the old code.
brooklyn!
433d40686d