Svelte 5: legacy `$:` behaves differently

[zeroberry]

Describe the bug

In Svelte 5 legacy reactive $: also runs before onMount if its dependency state is updated in the child component initialization. (via default value or assignment)
I think relying on this behavior is a bad practice, but it could be a breaking change.

<script>
	import { onMount } from 'svelte'
	import Component from "./Component.svelte"
	
	let foo;

	onMount(() => {
		foo = false;
	})

	$: if (foo) {
		// This now runs in svelte 5
		console.log('reactive: ', foo);
	}
</script>

<Component bind:foo />

<!-- Component.svelte -->
<script>
	export let foo = true;
</script>

Reproduction

Same code in Svelte 4 and Svelte 5 playgrounds:

• Svelte 4 REPL
• Svelte 5 REPL

Logs

System Info

-

Severity

blocking an upgrade

[dummdidumm]

Unless people rely on this in critical was I don't think we should change this and instead document it as a breaking change