fix (#8287)

Author: markdevocht

ios/BottomTabsAfterInitialTabAttacher.mm

@@ -9,6 +9,10 @@ - (void)attach:(UITabBarController *)bottomTabsController {
       for (UIViewController *viewController in bottomTabsController.deselectedViewControllers) {
         dispatch_async(dispatch_get_main_queue(), ^{
           UIWindow *preloadWindow = [[UIWindow alloc] initWithFrame:CGRectZero];
+          // Clip the preload window so the deselected tab's full-screen reactView,
+          // which is briefly hosted here while it renders, can't draw outside the
+          // zero-frame window and flicker over the already-visible selected tab.
+          preloadWindow.clipsToBounds = YES;
           preloadWindow.hidden = NO;
 
           dispatch_group_t ready = dispatch_group_create();

ios/BottomTabsTogetherAttacher.mm

@@ -7,6 +7,10 @@ - (void)attach:(RNNBottomTabsController *)bottomTabsController {
     dispatch_group_t ready = dispatch_group_create();
 
     UIWindow *preloadWindow = [[UIWindow alloc] initWithFrame:CGRectZero];
+    // Clip the preload window so the full-screen reactViews hosted here while
+    // they render can't draw outside the zero-frame window. Today the splash
+    // screen masks this, but without clipping it's a latent flicker.
+    preloadWindow.clipsToBounds = YES;
     preloadWindow.hidden = NO;
 
     NSMapTable *reactViewToParent = [NSMapTable strongToStrongObjectsMapTable];