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.
useBackToClose closes an overlay programmatically (X button or ESC) by
setting the module-global suppressPop=true and calling history.back(), so
the remaining overlays' popstate handlers ignore that synthetic pop. But
the flag was only ever reset by one of those handlers — when the closed
overlay had no overlay underneath it (the common single-player case),
nothing consumed it and suppressPop leaked true.
The next overlay's first real Back then hit the stale true, reset it and
returned early without closing — the Back was silently swallowed. Symptom
in the YouTube-search flow: close the player with the X button, open
another, press Back once -> nothing happens; a second Back closes it AND
discards the search results, dumping you on the normal feed.
Fix: register a one-shot popstate listener alongside suppressPop=true so
the flag is cleared on that very pop even when no overlay remains to
consume it. Nesting is unaffected (an underlying overlay still short-
circuits first and the one-shot is a redundant no-op).
In-app history only tracked the top-level page, so Back from a module sub-view
(e.g. a Messages thread) or with a modal open jumped straight to the previous
module. Add two history primitives: useHistorySubview (a module's sub-view rides
in history.state, so Back returns to its root first) and useBackToClose (a
mounted overlay occupies one history entry; Back closes the topmost, nesting-safe
so a button-close doesn't trip the modals underneath). Apply to the Messages page
views and to PlayerModal + the shared Modal. setPage now pushes a clean entry so
each page starts at its root.
