Publish the current queue head via a StateFlow so any host that subscribes
after a message is posted (e.g. a screen recomposing as a flow pops back to it)
still observes it. The previous mutex-gated flow delivered each message to a
single parked collector, which could starve the host actually on screen and
drop the snackbar. Adds a regression test for the late-subscriber case.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Jenna Vassar
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