[iOS] Have correct zPosition for grid cells

This CL makes sure to update the zPosition of the cells in the TabGrid
when they are inserted to allow the upper rows to be above the lower
rows.

Fixed: 1155122
Change-Id: Ib232dab6d35fa23c828fb027358cb2411e396982
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2571284Reviewed-by: Robbie Gibson <rkgibson@google.com>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833355}

ios/chrome/browser/ui/tab_switcher/tab_grid/grid/grid_view_controller.mm

@@ -369,9 +369,9 @@ NSIndexPath* CreateIndexPath(NSInteger index) {
     GridCell* gridCell = base::mac::ObjCCastStrict<GridCell>(cell);
     [self configureCell:gridCell withItem:item];
   }
-  // Set the z index of cells so that lower rows are superposed during
-  // transitions between grid and horizontal layouts.
-  cell.layer.zPosition = itemIndex;
+  // Set the z index of cells so that lower rows are moving behind the upper
+  // rows during transitions between grid and horizontal layouts.
+  cell.layer.zPosition = self.items.count - itemIndex;
 
 #if defined(__IPHONE_13_4)
   if (@available(iOS 13.4, *)) {
@@ -710,6 +710,8 @@ NSIndexPath* CreateIndexPath(NSInteger index) {
   [self performModelUpdates:modelUpdates
                 collectionViewUpdates:collectionViewUpdates
       collectionViewUpdatesCompletion:completion];
+
+  [self updateVisibleCellZIndex];
 }
 
 - (void)removeItemWithID:(NSString*)removedItemID
@@ -750,6 +752,8 @@ NSIndexPath* CreateIndexPath(NSInteger index) {
   [self performModelUpdates:modelUpdates
                 collectionViewUpdates:collectionViewUpdates
       collectionViewUpdatesCompletion:completion];
+
+  [self updateVisibleCellZIndex];
 }
 
 - (void)selectItemWithID:(NSString*)selectedItemID {
@@ -804,6 +808,8 @@ NSIndexPath* CreateIndexPath(NSInteger index) {
   [self performModelUpdates:modelUpdates
                 collectionViewUpdates:collectionViewUpdates
       collectionViewUpdatesCompletion:completion];
+
+  [self updateVisibleCellZIndex];
 }
 
 #pragma mark - LayoutSwitcher
@@ -1010,6 +1016,19 @@ NSIndexPath* CreateIndexPath(NSInteger index) {
       1 - offset / kScrollThresholdForPlusSignButtonHide, 0, 1);
 }
 
+// Updates the ZIndex of the visible cells to have the cells of the upper rows
+// be above the cell of the lower rows.
+- (void)updateVisibleCellZIndex {
+  for (NSIndexPath* indexPath in self.collectionView
+           .indexPathsForVisibleItems) {
+    // Set the z index of cells so that lower rows are moving behind the upper
+    // rows during transitions between grid and horizontal layouts.
+    UICollectionViewCell* cell =
+        [self.collectionView cellForItemAtIndexPath:indexPath];
+    cell.layer.zPosition = self.items.count - indexPath.item;
+  }
+}
+
 #pragma mark - Custom Gesture-based Reordering
 
 // Handle the long-press gesture used to reorder cells in the collection view.