svelte/documentation/docs/03-template-syntax/14-in-and-out.md

title: in: and out:

The in: and out: directives are identical to transition:, except that the resulting transitions are not bidirectional — an in transition will continue to 'play' alongside the out transition, rather than reversing, if the block is outroed while the transition is in progress. If an out transition is aborted, transitions will restart from scratch.

<script>
  import { fade, fly } from 'svelte/transition';
  
  let visible = $state(false);
</script>

<label>
  <input type="checkbox" bind:checked={visible}>
  visible
</label>

{#if visible}
	<div in:fly out:fade>flies in, fades out</div>
{/if}