fix(solid-virtual): preserve measurements on option updates

.changeset/quiet-cats-preserve.md

@@ -0,0 +1,5 @@
+---
+'@tanstack/solid-virtual': patch
+---
+
+Preserve measured item sizes when reactive virtualizer options change.

packages/solid-virtual/package.json

@@ -26,6 +26,8 @@
     "clean": "premove ./dist ./coverage",
     "test:eslint": "eslint ./src",
     "test:types": "tsc",
+    "test:lib": "vitest",
+    "test:lib:dev": "pnpm run test:lib --watch",
     "test:build": "publint --strict",
     "build": "vite build"
   },

packages/solid-virtual/src/index.tsx

@@ -81,7 +81,9 @@ function createVirtualizerBase<
         },
       }),
     )
-    virtualizer.measure()
+    virtualizer._willUpdate()
+    setVirtualItems(reconcile(instance.getVirtualItems(), { key: 'index' }))
+    setTotalSize(instance.getTotalSize())
   })
 
   return virtualizer

packages/solid-virtual/tests/index.test.ts

@@ -0,0 +1,28 @@
+import { expect, test } from 'vitest'
+import { createRoot, createSignal } from 'solid-js'
+
+import { createVirtualizer } from '../src/index'
+
+test('preserves measured sizes when reactive options change', () => {
+  createRoot((dispose) => {
+    const [count, setCount] = createSignal(2)
+    const virtualizer = createVirtualizer<HTMLDivElement, HTMLDivElement>({
+      get count() {
+        return count()
+      },
+      getScrollElement: () => null,
+      estimateSize: () => 60,
+      initialRect: { width: 800, height: 600 },
+    })
+
+    expect(virtualizer.getTotalSize()).toBe(120)
+    virtualizer.resizeItem(0, 100)
+    expect(virtualizer.getTotalSize()).toBe(160)
+
+    setCount(3)
+
+    expect(virtualizer.itemSizeCache.get(0)).toBe(100)
+    expect(virtualizer.getTotalSize()).toBe(220)
+    dispose()
+  })
+})