From 85d680582b5435c4dbe93db7c6c28d98685f2b47 Mon Sep 17 00:00:00 2001
From: Vladislav Logvin <122863429+Black1358@users.noreply.github.com>
Date: Tue, 7 May 2024 15:44:49 +0300
Subject: [PATCH] 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                |  5 ++++
 .../client/dom/elements/bindings/window.js    |  4 ++-
 .../window-bind-scroll-update-2/Child.svelte  |  5 ++++
 .../window-bind-scroll-update-2/_config.js    | 30 +++++++++++++++++++
 .../window-bind-scroll-update-2/main.svelte   | 13 ++++++++
 5 files changed, 56 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/green-fishes-lie.md
 create mode 100644 packages/svelte/tests/runtime-legacy/samples/window-bind-scroll-update-2/Child.svelte
 create mode 100644 packages/svelte/tests/runtime-legacy/samples/window-bind-scroll-update-2/_config.js
 create mode 100644 packages/svelte/tests/runtime-legacy/samples/window-bind-scroll-update-2/main.svelte

diff --git a/.changeset/green-fishes-lie.md b/.changeset/green-fishes-lie.md
new file mode 100644
index 0000000000..2d890d16d7
--- /dev/null
+++ b/.changeset/green-fishes-lie.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: only initiate scroll if scroll binding has existing value
diff --git a/packages/svelte/src/internal/client/dom/elements/bindings/window.js b/packages/svelte/src/internal/client/dom/elements/bindings/window.js
index 8cab87122b..55c0b645de 100644
--- a/packages/svelte/src/internal/client/dom/elements/bindings/window.js
+++ b/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);
diff --git a/packages/svelte/tests/runtime-legacy/samples/window-bind-scroll-update-2/Child.svelte b/packages/svelte/tests/runtime-legacy/samples/window-bind-scroll-update-2/Child.svelte
new file mode 100644
index 0000000000..952d3dd1ca
--- /dev/null
+++ b/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/>
diff --git a/packages/svelte/tests/runtime-legacy/samples/window-bind-scroll-update-2/_config.js b/packages/svelte/tests/runtime-legacy/samples/window-bind-scroll-update-2/_config.js
new file mode 100644
index 0000000000..29100e3e70
--- /dev/null
+++ b/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);
+	}
+});
diff --git a/packages/svelte/tests/runtime-legacy/samples/window-bind-scroll-update-2/main.svelte b/packages/svelte/tests/runtime-legacy/samples/window-bind-scroll-update-2/main.svelte
new file mode 100644
index 0000000000..9b3de0b307
--- /dev/null
+++ b/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}