Bug: ViewTransition onExit cleanup not called on unmount

[rickhanlonii]

Noticed this while writing docs:

<ViewTransition
  onEnter={(instance) => {
    const anim = instance.new.animate(/* ... */);
    return () => {
      anim.cancel();
    };
  }}
  onExit={(instance) => {
    const anim = instance.new.animate(/* ... */);
    return () => {
      anim.cancel();
    };
  }}
>
  {/* ... */}    
</ViewTransition>

If you have a pendingonEnter animation and the tree is unmounted, the onEnter cleanup is called immediately.

But if you have a pending onExit and the tree is unmounted, the onExit cleanup isn't called until the animation completes.

This means you can't cancel the animation:

Screen.Recording.2026-02-21.at.11.11.57.AM.mov

Sandbox: https://codesandbox.io/p/sandbox/naughty-wu-pfkh2m

[darshjme-codes]

Triage note:

Bug: viewTransition.onExit cleanup function not called when component unmounts.

Labels applied: Status: Unconfirmed ✓

Suggested additional labels:

• Component: DOM
• Browser Specific (View Transitions API)

Expected: Cleanup functions should run on unmount
Actual: onExit cleanup not triggered

This may be related to View Transitions lifecycle not properly integrating with React's cleanup phase.