Fix: bind:scroll resets scroll state (#11469)

* Fixed: bind:scroll resets scroll state

* failing test

* bail if value is undefined

* changeset

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>

.changeset/green-fishes-lie.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: only initiate scroll if scroll binding has existing value

packages/svelte/src/internal/client/dom/elements/bindings/window.js

@@ -32,7 +32,9 @@ export function bind_window_scroll(type, get_value, update) {
 	};
 
 	render_effect(() => {
-		latest_value = get_value() || 0;
+		latest_value = get_value();
+		if (latest_value === undefined) return;
+
 		if (!scrolling) {
 			scrolling = true;
 			clearTimeout(timeout);

packages/svelte/tests/runtime-legacy/samples/window-bind-scroll-update-2/Child.svelte

@@ -0,0 +1,5 @@
+<script>
+	let scrollY;
+</script>
+
+<svelte:window bind:scrollY/>

packages/svelte/tests/runtime-legacy/samples/window-bind-scroll-update-2/_config.js

@@ -0,0 +1,30 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+/** @type {Window['scrollTo']} */
+let original_scrollTo;
+
+export default test({
+	before_test() {
+		original_scrollTo = window.scrollTo;
+
+		// @ts-ignore
+		window.scrollTo = (x, y) => {
+			window.scrollX = x;
+			window.scrollY = y;
+		};
+	},
+
+	after_test() {
+		window.scrollTo = original_scrollTo;
+	},
+
+	async test({ assert, component, window, target }) {
+		window.scrollTo(0, 500);
+
+		const button = target.querySelector('button');
+		flushSync(() => button?.click());
+
+		assert.equal(window.scrollY, 500);
+	}
+});

packages/svelte/tests/runtime-legacy/samples/window-bind-scroll-update-2/main.svelte

@@ -0,0 +1,13 @@
+<script>
+	import Child from './Child.svelte';
+
+	let show = false;
+</script>
+
+<div style='width: 100%; height: 9999px;'></div>
+
+<button on:click={() => show = !show}>toggle</button>
+
+{#if show}
+	<Child />
+{/if}