[tab_grid] Hack-in border for active tab.

ios/clean/chrome/browser/ui/tab_grid/tab_grid_view_controller.mm

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // ======                        New Architecture                         =====
 // =         This code is only used in the new iOS Chrome architecture.       =
 // ============================================================================
 
 #import "ios/clean/chrome/browser/ui/tab_grid/tab_grid_view_controller.h"
 
@@ skipping 100 matching lines @@
   TabSwitcherLocalSessionCell* cell =
       base::mac::ObjCCastStrict<TabSwitcherLocalSessionCell>([collectionView
           dequeueReusableCellWithReuseIdentifier:
                               [TabSwitcherLocalSessionCell identifier]
                                     forIndexPath:indexPath]);
   cell.delegate = self;
   [cell setSessionType:TabSwitcherSessionType::REGULAR_SESSION];
   [cell setAppearanceForTabTitle:[self.dataSource titleAtIndex:indexPath.item]
                          favicon:nil
                         cellSize:CGSizeZero];
+  [cell setSelected:(indexPath.item == [self.dataSource indexOfActiveTab])];
   return cell;
 }
 
+#pragma mark - UICollectionViewDelegate methods
+
+- (BOOL)collectionView:(UICollectionView*)collectionView
+    shouldSelectItemAtIndexPath:(NSIndexPath*)indexPath {
+  // Prevent user selection of items.
+  return NO;
+}
+
 #pragma mark - ZoomTransitionDelegate methods
 
 - (CGRect)rectForZoomWithKey:(NSObject*)key inView:(UIView*)view {
   NSIndexPath* cellPath = base::mac::ObjCCastStrict<NSIndexPath>(key);
   if (!key)
     return CGRectNull;
   UICollectionViewCell* cell = [self.grid cellForItemAtIndexPath:cellPath];
   return [view convertRect:cell.bounds fromView:cell];
 }
 
 #pragma mark - SettingsActions
 
 - (void)showSettings:(id)sender {
   [self.settingsCommandHandler showSettings];
 }
 
 #pragma mark - TabGridActions
 
 - (void)showTabGrid:(id)sender {
   [self.tabGridCommandHandler showTabGrid];
 }
 
 - (void)createNewTab:(id)sender {
   // PLACEHOLDER: The new WebStateList data structure will have implications
   // on how new tabs are created.
   NSInteger index = [self.grid numberOfItemsInSection:0];
   NSIndexPath* indexPath = [NSIndexPath indexPathForItem:index inSection:0];
   auto updateBlock = ^{
+    // Unselect current selected item.
+    NSInteger selectedIndex = [self.dataSource indexOfActiveTab];
+    NSIndexPath* selectedIndexPath =
+        [NSIndexPath indexPathForItem:selectedIndex inSection:0];
+    [self.grid reloadItemsAtIndexPaths:@[ selectedIndexPath ]];
+
+    // Create and show new tab.
     [self.tabCommandHandler createNewTabAtIndexPath:indexPath];
     [self.tabCommandHandler showTabAtIndexPath:indexPath];
     [self.grid insertItemsAtIndexPaths:@[ indexPath ]];
   };
   [self.grid performBatchUpdates:updateBlock completion:nil];
 }
 
 #pragma mark - SessionCellDelegate
 
 - (TabSwitcherCache*)tabSwitcherCache {
   // PLACEHOLDER: return image cache.
   return nil;
 }
 
 - (void)cellPressed:(UICollectionViewCell*)cell {
-  [self.tabCommandHandler showTabAtIndexPath:[self.grid indexPathForCell:cell]];
+  NSInteger selectedIndex = [self.dataSource indexOfActiveTab];
+  NSIndexPath* selectedIndexPath =
+      [NSIndexPath indexPathForItem:selectedIndex inSection:0];
+
+  NSIndexPath* newSelectedIndexPath = [self.grid indexPathForCell:cell];
+  [self.tabCommandHandler showTabAtIndexPath:newSelectedIndexPath];
+  [self.grid
+      reloadItemsAtIndexPaths:@[ selectedIndexPath, newSelectedIndexPath ]];
 }
 
 - (void)deleteButtonPressedForCell:(UICollectionViewCell*)cell {
   auto updateBlock = ^{
     NSIndexPath* indexPath = [self.grid indexPathForCell:cell];
     [self.tabCommandHandler closeTabAtIndexPath:indexPath];
     [self.grid deleteItemsAtIndexPaths:@[ indexPath ]];
   };
   [self.grid performBatchUpdates:updateBlock completion:nil];
 }
 
 @end