`svelty-picker` value desync bug since 5.36.0

[longnguyen2004]

Describe the bug

I found a regression somewhere between 5.20.4 and the current version, that breaks the svelty-picker package. In the reproduction below, trying to click on the time picker too quickly will eventually cause the stored time value to desync from the value displayed in the picker, and the picker itself will also cease to work properly. Pinning the Svelte version to 5.20.4 fixes the issue. Notably, I can only reproduce this in a SvelteKit project, it doesn't occur in the Svelte REPL.

Reproduction

https://stackblitz.com/edit/svelty-picker-desync

Click randomly on the time picker and observe the time desync issue. Downgrade to 5.20.4 and observe that the issue is gone

Logs

System Info

System:
    OS: Linux 5.0 undefined
    CPU: (8) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Memory: 0 Bytes / 0 Bytes
    Shell: 1.0 - /bin/jsh
  Binaries:
    Node: 20.19.1 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 10.8.2 - /usr/local/bin/npm
    pnpm: 8.15.6 - /usr/local/bin/pnpm
  npmPackages:
    svelte: ^5.25.0 => 5.39.4

Severity

blocking all usage of svelte

[longnguyen2004]

Manually bisected and found the problematic version to be 5.36.0. This is also the version where #15844 is introduced

[dummdidumm]

This isn't reproducible in the REPL because the REPL is always in async mode. So this is a bug in non-async mode. Copying this

<script lang="ts">
	import SveltyPicker from "svelty-picker"

	let time = $state("00:00");
</script>

<div>{time}</div>
<SveltyPicker bind:value={time} format="hh:ii" pickerOnly/>

into the playground locally and setting experimental { async: false } makes it reproducible