Fix: hidden-document view transition unhandledrejection

Issue #3989 · PR #4044 · Base: 7f9160f0

Before Fix

four-dev @ 7f9160f0

Swap still happens, but ready rejection is unhandled.

Click Swap…

After Fix

7f9160f0 + fix

Same skip, no unhandledrejection.

Click Swap…
Mock startViewTransition rejects ready the way a hidden document does. Left: unhandled InvalidStateError. Right: vt.ready.catch(() => {}) before awaiting finished.