Code Quality: Avoid generation of superfluous CollectionViewSource (#14629)

Author: hishitetsu

src/Files.App/Views/Layouts/BaseLayoutPage.cs

@@ -1287,28 +1287,31 @@ protected void ItemsLayout_Drop(object sender, DragEventArgs e)
 			CommandsViewModel?.DropCommand?.Execute(e);
 		}
 
+		private CollectionViewSource _CollectionViewSource;
+		private CollectionViewSource _GroupedCollectionViewSource;
+
 		private void UpdateCollectionViewSource()
 		{
 			if (ParentShellPageInstance is null)
 				return;
 
 			if (ParentShellPageInstance.FilesystemViewModel.FilesAndFolders.IsGrouped)
 			{
-				var newSource = new CollectionViewSource()
+				_GroupedCollectionViewSource ??= new CollectionViewSource()
 				{
-					IsSourceGrouped = true,
-					Source = ParentShellPageInstance.FilesystemViewModel.FilesAndFolders.GroupedCollection
+					IsSourceGrouped = true
 				};
-				CollectionViewSource = newSource;
+				_GroupedCollectionViewSource.Source = ParentShellPageInstance.FilesystemViewModel.FilesAndFolders.GroupedCollection;
+				CollectionViewSource = _GroupedCollectionViewSource;
 			}
 			else
 			{
-				var newSource = new CollectionViewSource()
+				_CollectionViewSource ??= new CollectionViewSource()
 				{
 					IsSourceGrouped = false,
 					Source = ParentShellPageInstance.FilesystemViewModel.FilesAndFolders
 				};
-				CollectionViewSource = newSource;
+				CollectionViewSource = _CollectionViewSource;
 			}
 		}