useBackToClose eagerly pushed a history entry on mount and called history.back()
on unmount. During a modal->modal handoff (e.g. About -> Release Notes) the two ran
interleaved in one React commit, so the entering modal's popstate listener mistook the
leaving modal's back() for a genuine user Back and closed itself instantly. It also left
the history pointer behind the surviving entry, so a later browser Back walked off the app.
Replace the eager per-mount push/pop with a single shared popstate handler plus a
coalesced microtask that reconciles history depth to the live overlay count once per tick.
A handoff's -1/+1 nets to zero, so the new modal simply reuses the old entry -- no churn,
no flash, and Back closes the modal in-app.
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