From 65d055b100cbf65f8b011a281c8af143d5557429 Mon Sep 17 00:00:00 2001
From: clean99 <53188310+clean99@users.noreply.github.com>
Date: Wed, 28 Oct 2026 00:53:37 +0800
Subject: [PATCH] fix(svelte-virtual): preserve onChange across partial option
 updates

---
 .changeset/tidy-scroll-callbacks.md         |   5 +
 packages/svelte-virtual/package.json        |   1 +
 packages/svelte-virtual/src/index.ts        |   4 +-
 packages/svelte-virtual/tests/index.test.ts | 118 ++++++++++++++++++++
 packages/svelte-virtual/tsconfig.json       |   1 +
 packages/svelte-virtual/vite.config.ts      |   4 +
 6 files changed, 132 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/tidy-scroll-callbacks.md
 create mode 100644 packages/svelte-virtual/tests/index.test.ts

diff --git a/.changeset/tidy-scroll-callbacks.md b/.changeset/tidy-scroll-callbacks.md
new file mode 100644
index 000000000..239885dcd
--- /dev/null
+++ b/.changeset/tidy-scroll-callbacks.md
@@ -0,0 +1,5 @@
+---
+'@tanstack/svelte-virtual': patch
+---
+
+Preserve the `onChange` callback when partial `setOptions` updates omit it.
diff --git a/packages/svelte-virtual/package.json b/packages/svelte-virtual/package.json
index 821427816..a690f45a8 100644
--- a/packages/svelte-virtual/package.json
+++ b/packages/svelte-virtual/package.json
@@ -28,6 +28,7 @@
     "test:eslint": "eslint ./src",
     "test:types": "tsc",
     "test:build": "publint --strict",
+    "test:lib": "vitest",
     "build": "svelte-package --input ./src --output ./dist"
   },
   "type": "module",
diff --git a/packages/svelte-virtual/src/index.ts b/packages/svelte-virtual/src/index.ts
index 1607d8c2c..18b1a62f0 100644
--- a/packages/svelte-virtual/src/index.ts
+++ b/packages/svelte-virtual/src/index.ts
@@ -30,6 +30,7 @@ function createVirtualizerBase<
 ): Readable<SvelteVirtualizer<TScrollElement, TItemElement>> {
   const virtualizer = new Virtualizer(initialOptions)
   const originalSetOptions = virtualizer.setOptions
+  let onChange = initialOptions.onChange
 
   // eslint-disable-next-line prefer-const
   let virtualizerWritable: Writable<Virtualizer<TScrollElement, TItemElement>>
@@ -39,9 +40,10 @@ function createVirtualizerBase<
   ) => {
     const resolvedOptions = {
       ...virtualizer.options,
+      onChange,
       ...options,
-      onChange: options.onChange,
     }
+    onChange = resolvedOptions.onChange
     originalSetOptions({
       ...resolvedOptions,
       onChange: (
diff --git a/packages/svelte-virtual/tests/index.test.ts b/packages/svelte-virtual/tests/index.test.ts
new file mode 100644
index 000000000..91ebc4259
--- /dev/null
+++ b/packages/svelte-virtual/tests/index.test.ts
@@ -0,0 +1,118 @@
+import { get } from 'svelte/store'
+import { describe, expect, test, vi } from 'vitest'
+import { createVirtualizer, createWindowVirtualizer } from '../src'
+import type { SvelteVirtualizer } from '../src'
+
+type TestVirtualizer =
+  | SvelteVirtualizer<Element, Element>
+  | SvelteVirtualizer<Window, Element>
+
+describe.each([
+  ['createVirtualizer', createVirtualizer<Element, Element>],
+  ['createWindowVirtualizer', createWindowVirtualizer<Element>],
+] as const)('%s', (_name, create) => {
+  test('preserves onChange across partial option updates', () => {
+    const onChange = vi.fn()
+    const subscriber = vi.fn()
+    const store = create({
+      count: 100,
+      estimateSize: () => 40,
+      getScrollElement: () => null,
+      onChange,
+    })
+    const unsubscribe = store.subscribe(subscriber)
+    const virtualizer = get<TestVirtualizer>(store)
+
+    try {
+      for (const options of [{ count: 110 }, { overscan: 2 }, {}]) {
+        subscriber.mockClear()
+        virtualizer.setOptions(options)
+        expect(subscriber).toHaveBeenCalledOnce()
+        expect(virtualizer.options.count).toBe(110)
+        expect(virtualizer.getTotalSize()).toBe(4400)
+
+        onChange.mockClear()
+        subscriber.mockClear()
+        virtualizer.measure()
+
+        expect(onChange).toHaveBeenCalledExactlyOnceWith(virtualizer, false)
+        expect(subscriber).toHaveBeenCalledExactlyOnceWith(virtualizer)
+      }
+    } finally {
+      unsubscribe()
+    }
+  })
+
+  test('replaces onChange without retaining the previous wrapper', () => {
+    const onChange = vi.fn()
+    const nextOnChange = vi.fn()
+    const store = create({
+      count: 100,
+      estimateSize: () => 40,
+      getScrollElement: () => null,
+      onChange,
+    })
+    const unsubscribe = store.subscribe(() => {})
+    const virtualizer = get<TestVirtualizer>(store)
+
+    try {
+      virtualizer.setOptions({ onChange: nextOnChange })
+      virtualizer.setOptions({ count: 110 })
+      onChange.mockClear()
+      nextOnChange.mockClear()
+      virtualizer.measure()
+
+      expect(onChange).not.toHaveBeenCalled()
+      expect(nextOnChange).toHaveBeenCalledExactlyOnceWith(virtualizer, false)
+    } finally {
+      unsubscribe()
+    }
+  })
+
+  test('can clear onChange without stopping store updates', () => {
+    const onChange = vi.fn()
+    const subscriber = vi.fn()
+    const store = create({
+      count: 100,
+      estimateSize: () => 40,
+      getScrollElement: () => null,
+      onChange,
+    })
+    const unsubscribe = store.subscribe(subscriber)
+    const virtualizer = get<TestVirtualizer>(store)
+
+    try {
+      virtualizer.setOptions({ onChange: undefined })
+      virtualizer.setOptions({ count: 110 })
+      onChange.mockClear()
+      subscriber.mockClear()
+      virtualizer.measure()
+
+      expect(onChange).not.toHaveBeenCalled()
+      expect(subscriber).toHaveBeenCalledExactlyOnceWith(virtualizer)
+    } finally {
+      unsubscribe()
+    }
+  })
+
+  test('updates the store when no onChange was supplied', () => {
+    const subscriber = vi.fn()
+    const store = create({
+      count: 100,
+      estimateSize: () => 40,
+      getScrollElement: () => null,
+    })
+    const unsubscribe = store.subscribe(subscriber)
+    const virtualizer = get<TestVirtualizer>(store)
+
+    try {
+      virtualizer.setOptions({ count: 110 })
+      subscriber.mockClear()
+      virtualizer.measure()
+
+      expect(subscriber).toHaveBeenCalledExactlyOnceWith(virtualizer)
+    } finally {
+      unsubscribe()
+    }
+  })
+})
diff --git a/packages/svelte-virtual/tsconfig.json b/packages/svelte-virtual/tsconfig.json
index 83321eef4..24b25061a 100644
--- a/packages/svelte-virtual/tsconfig.json
+++ b/packages/svelte-virtual/tsconfig.json
@@ -4,6 +4,7 @@
     "src/**/*.js",
     "src/**/*.ts",
     "src/**/*.svelte",
+    "tests/**/*.ts",
     "svelte.config.js",
     "vite.config.ts"
   ]
diff --git a/packages/svelte-virtual/vite.config.ts b/packages/svelte-virtual/vite.config.ts
index 3d696d6ec..32f16fb77 100644
--- a/packages/svelte-virtual/vite.config.ts
+++ b/packages/svelte-virtual/vite.config.ts
@@ -3,4 +3,8 @@ import { defineConfig } from 'vitest/config'
 
 export default defineConfig({
   plugins: [svelte()],
+  test: {
+    dir: './tests',
+    watch: false,
+  },
 })