Needed for #12860.
If the admin panel broadcasts foreign postMessage events, i.e. those
without the `payload-live-preview` signature, client-side live preview
subscriptions will reset back to initial state.
This is because we dispatch two postMessage events in the admin panel,
one for client-side live preview to catch (`payload-live-preview`), and
the other for server-side live preview (`payload-document-event`). This
was not previously noticeable because both events would only get called
simultaneously on initial render, where initial state is already the
expected result.
Now that Live Preview can be freely toggled on and off, both events are
frequently dispatched and very obviously disregard the current working
state.
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67fa5a0b3b