fix: ensure window scroll binding inits to window value

.changeset/gorgeous-mails-roll.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: ensure window scroll binding inits to window value

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

@@ -42,6 +42,8 @@ export function bind_window_scroll(type, get_value, update) {
 				scrollTo(window.scrollX, latest_value);
 			}
 			timeout = setTimeout(clear, 100);
+		} else if (latest_value == null) {
+			update(window[is_scrolling_x ? 'scrollX' : 'scrollY']);
 		}
 	});
 

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

@@ -0,0 +1,29 @@
+import { test } from '../../test';
+import { vi } from 'vitest';
+
+/** @type {Window['scrollTo']} */
+let original_scrollTo;
+
+export default test({
+	before_test() {
+		vi.useFakeTimers();
+
+		Object.defineProperties(window, {
+			scrollY: {
+				value: 0,
+				configurable: true,
+				writable: true
+			}
+		});
+	},
+
+	after_test() {
+		vi.useRealTimers();
+	},
+
+	async test({ assert, component, window }) {
+		assert.equal(window.pageYOffset, 0);
+
+		assert.equal(component.scrollY, 0);
+	}
+});

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

@@ -0,0 +1,7 @@
+<script>
+	export let scrollY;
+	$: console.log(scrollY);
+</script>
+
+<svelte:window bind:scrollY />
+<div style="height: 200vh; background-image: linear-gradient(green, pink)"></div>