ref(onboarding): Mock virtualizer in repo selector tests

Use the same @tanstack/react-virtual mock as scmPlatformFeatures
tests instead of a runtime fallback in the component. Reverts the
jsdom fallback in ScmVirtualizedMenuList.

Author: jaydgoss

static/app/views/onboarding/components/scmRepoSelector.spec.tsx

@@ -11,6 +11,21 @@ import {
 
 import {ScmRepoSelector} from './scmRepoSelector';
 
+// Mock the virtualizer so all items render in JSDOM (no layout engine).
+jest.mock('@tanstack/react-virtual', () => ({
+  useVirtualizer: jest.fn(({count}) => ({
+    getVirtualItems: () =>
+      Array.from({length: count}, (_, i) => ({
+        key: i,
+        index: i,
+        start: i * 36,
+        size: 36,
+      })),
+    getTotalSize: () => count * 36,
+    measureElement: jest.fn(),
+  })),
+}));
+
 function makeOnboardingWrapper(initialState?: OnboardingSessionState) {
   return function OnboardingWrapper({children}: {children?: React.ReactNode}) {
     return (

static/app/views/onboarding/components/scmVirtualizedMenuList.tsx

@@ -55,15 +55,6 @@ export function ScmVirtualizedMenuList({
     );
   }
 
-  // Fallback when the virtualizer can't measure the container (e.g. jsdom)
-  if (virtualItems.length === 0 && items.length > 0) {
-    return (
-      <div ref={combinedRef} {...innerProps} style={{maxHeight, overflowY: 'auto'}}>
-        {children}
-      </div>
-    );
-  }
-
   return (
     <div ref={combinedRef} {...innerProps} style={{maxHeight, overflowY: 'auto'}}>
       <div style={{height: virtualizer.getTotalSize(), position: 'relative'}}>