transition: and indexed {#each} block race condition

[GauBen]

Describe the bug

I found a very weird edge case that only seems to happen when a lot of unusual conditions align (indexed each, transitions, timings, focus...).

Here is a very minimal "autocomplete" implementation:

When one suggestion is clicked, new suggestions appear by calling this replacement snippet list.set(['three', 'four'])

expected:

actual:

Old suggestions are not properly removed from the DOM

Reproduction

https://svelte.dev/playground/1841f931f328483283516f946b1a44e0?version=5.1.2

Very small changes can make it work properly:

• Moving onfocusin to the <input/> element
• Removing the index in the {#each block}
• Removing the 4ms delay in updateList

Logs

No response

System Info

Svelte 5.1.2

Severity

annoyance

[GauBen]

After a quick investigation, the bug appeared in 5.0.0-next.269 (https://github.com/sveltejs/svelte/blob/main/packages/svelte/CHANGELOG.md#500-next269), it works fine before this release

[adiguba]

The PR #13646 is realy simple and only affect dynamic component, which are not used in your example.

Also, the bug seem already present on version 5.0.0-next.268 :
https://svelte.dev/playground/1841f931f328483283516f946b1a44e0?version=5.0.0-next.268

Using the REPL, I see that it was rather introduced in 5.0.0-next.263 :
https://svelte.dev/playground/1841f931f328483283516f946b1a44e0?version=5.0.0-next.262 => OK
https://svelte.dev/playground/1841f931f328483283516f946b1a44e0?version=5.0.0-next.263 => BUG

[trueadm]

Maybe it was #13527?

[GauBen]

Thank you a lot for your help @adiguba! Somehow in my original SvelteKit app the bug started to appear on a later Svelte version