A hidden session's live Browser stays mounted so returning shows the same
page, but it kept playing audio into the chat the user had switched to. The
parked-pane set is now an atom; a preview pane whose tab is parked mutes its
guest (webview.setAudioMuted) and unmutes on return only if it was the one that
muted it, so a page muted beforehand stays muted. Inactive tabs in the session
on screen keep playing, like background tabs in a browser.
Timers/rendering: a CSS-hidden webview still reports visible to Chromium and
Electron has no public API to mark a guest hidden without detaching it (any
detach destroys the guest, losing its state), so throttling is left as is.
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