Review follow-up. trackFocus stopped focusin propagation before asking
focusSelectedComposer to redirect; when the selection guard refused, the
element that kept focus had its focusin hidden from React and other root
listeners. Evaluate the guard first and leave the event alone when the
redirect is refused. The guard also resolved the anchor through
parentElement, so a range anchored at the editor element itself was
classified as outside the composer. DESIGN.md now states the rule.
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