`<svelte:boundary>` async reactivity loss

[dylan1951]

Describe the bug

Since 5.43.3 there appears to be some kind of race condition where the rendered value in the boundary can be stale.

Bug seems to have been introduced by #17098

Reproduction

REPL

<script>
    let foo = $state(1);
    let bar = $state(false);

    $effect(() => {
        foo = 2;
        bar = true;
    });

    async function stall() {
        return new Promise(resolve => setTimeout(resolve, 0));
    }

    let baz = $derived(bar ? await stall() : null);
</script>

<p>Outside boundary value: {foo}</p>

<svelte:boundary>
    {#snippet pending()}
        <p>Loading</p>
    {/snippet}

    {@const quux = await stall()}

    <p>Inside boundary value: {foo}</p>
</svelte:boundary>

Logs

System Info

Svelte playground

Severity

blocking an upgrade