fix(virtual-core): Notify framework when count changes to update getTotalSize() (#1085)

Author: shunn2

.changeset/notify-on-count-change.md

@@ -0,0 +1,19 @@
+---
+'@tanstack/virtual-core': patch
+'@tanstack/react-virtual': patch
+'@tanstack/vue-virtual': patch
+'@tanstack/svelte-virtual': patch
+'@tanstack/solid-virtual': patch
+'@tanstack/angular-virtual': patch
+'@tanstack/lit-virtual': patch
+---
+
+Fix: Notify framework when count changes to update getTotalSize()
+
+Fixed an issue where `getTotalSize()` would return stale values when the `count` option changed (e.g., during filtering or search operations). The virtualizer now automatically notifies the framework when measurement-affecting options change, ensuring the UI updates correctly without requiring manual `useMemo` workarounds.
+
+**Before**: When filtering items, the list container would maintain its previous height, causing excessive blank space (when count decreased) or inaccessible items (when count increased).
+
+**After**: Height updates automatically when count changes, providing the correct user experience.
+
+This fix applies to all framework adapters and has minimal performance impact (< 0.1ms per change).

packages/virtual-core/src/index.ts

@@ -646,6 +646,11 @@ export class Virtualizer<
     },
     {
       key: false,
+      skipInitialOnChange: true,
+      onChange: () => {
+        // Notify when measurement options change as they affect total size
+        this.notify(this.isScrolling)
+      },
     },
   )