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

[angular-bootstrap]

I’d like to take this issue.

I reproduced the behavior where onExit cleanup is deferred until transition finish when the tree unmounts. I’m working on a fix plus a regression test in ReactDOMViewTransition-test to ensure onExit cleanup runs immediately on unmount.

I’ll open a PR shortly and link it here.

[angular-bootstrap]

Checked the bundle-size report: increases are very small (roughly +0.04% to +0.10%, including gzip deltas) and below the “significant changes” threshold.

This is expected from the runtime-side fix to ViewTransition event cleanup handling (onExit cleanup-on-unmount path) plus the regression test updates.