fix: measure elements before taking siblings out of the flow

[Rich-Harris]

This reverts #11208 and implements position fixing in a slightly more logical way that also fixes #10251. It's still not perfect, — it's possible to cause short-lived glitches if you have lots of transitions and animations rapidly interacting — but it's better than Svelte 4. We can worry about perfection another time (possibly with a redesigned primitive).

As with #11208, no test because you can't really test this stuff in JSDOM.

This also fixes #11156 and #11199, by removing animations once they have completed.

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[changeset-bot]

🦋 Changeset detected

Latest commit: 8276f45

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

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svelte	Patch

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[Rich-Harris]

Converted to draft because I forgot to undelete the reverted changeset and add a new one - will do that when at my laptop