Edits wait up to 3s in a buffer before they are sent. Closing the
laptop or switching away right after an edit could leave that batch
unsent, and a crash or a discarded tab then lost it.
The batch is now sent at once when the tab is hidden (another tab, a
minimized window, a closed tab or laptop) or the window loses focus
(another app or a plugin iframe). The 3s window itself is unchanged.
pagehide and beforeunload are left out: they come too late for an
async save, and the tab is already hidden by then.
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