fix: Abort outro when intro starts

[Fruup]

Fixes #12319.

Currently, if an intro animation starts before an outro is finished, the outro overlaps the intro and leaves the element in an unwanted state (see the PR for a video). I think the outro should always be cancelled if an intro starts.

Reproduction

Write this into playgrounds/demo/src/main.svelte...

<!-- file: playgrounds/demo/src/main.svelte -->

<script lang="ts">
  import { fly } from 'svelte/transition';

  let state = $state(0);
</script>

<button onclick={() => state++}>{state}</button>

{#if state % 2 === 0}
  <h2 transition:fly={{ duration: 1000, x: 100 }}>Hello World!</h2>
{/if}

then...

cd playgrounds/demo
pnpm dev

---

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[changeset-bot]

🦋 Changeset detected

Latest commit: f0c2b45

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[trueadm]

Thanks for the fix. I tried it out locally and it does indeed improve the behaviour as mentioned.

[Rich-Harris]

thank you — we'll need a test for this before we can merge it. will try and add one later if no-one beats me to it

[dummdidumm]

Adjusted an existing test which turned out to test nothing since Svelte 4, because we changed transitions to be local by default back then; and because it was nested inside an each block it did nothing.

test added