fix: prevent numerous transition/animation memory leaks

[trueadm]

Whilst digging into #12719, I discovered several memory leaks where we retain DOM elements longer than we should. We also frustratingly retain the animation even after the DOM element has been detached and the animation is cancelled – but this seems to happen on the MDN site too, so maybe WAAPI has issues on Chromium?

I can't say for sure if this solves all the issues, but it definitely solves a whole load for me and memory usage now doesn't grow at a large rate when toggling the DOM elements in and out in the example.

Test case

• Go to the dev tools memory tab, record a snapshot
• Toggle the animation on and off
• Take another snapshot
• Toggle the animation on and off one last time
• Take a third snapshot

If you compare the 2nd and 3rd snapshots, we have no retained DOM elements leaking.

[changeset-bot]

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Latest commit: 9dbd96a

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[Rich-Harris]

Do you have a repro that demonstrates the leak on main being fixed with this PR? Since this stuff is hard to test for, it'd be good to link to something in the comments or we're liable to regress

[trueadm]

Do you have a repro that demonstrates the leak on main being fixed with this PR? Since this stuff is hard to test for, it'd be good to link to something in the comments or we're liable to regress

I added the test case in the OP.

[trueadm]

I updated the PR. @Rich-Harris it seems that we don't need to null those fields, the reason for the retainment was the unmount function being stored on the window. The other fixes are still valid