New gallery performance improvements (#17167)

examples/flutter_gallery/lib/gallery/home.dart

@@ -52,7 +52,10 @@ class _CategoryItem extends StatelessWidget {
     final ThemeData theme = Theme.of(context);
     final bool isDark = theme.brightness == Brightness.dark;
 
-    return new RawMaterialButton(
+    // This repaint boundary prevents the entire _CategoriesPage from being
+    // repainted when the button's ink splash animates.
+    return new RepaintBoundary(
+      child: new RawMaterialButton(
         padding: EdgeInsets.zero,
         splashColor: theme.primaryColor.withOpacity(0.12),
         highlightColor: Colors.transparent,
@@ -84,6 +87,7 @@ class _CategoryItem extends StatelessWidget {
             ),
           ],
         ),
+      ),
     );
   }
 }
@@ -112,7 +116,11 @@ class _CategoriesPage extends StatelessWidget {
           final double rowHeight = columnWidth * aspectRatio;
           final int rowCount = (categories.length + columnCount - 1) ~/ columnCount;
 
-          return new Column(
+          // This repaint boundary prevents the inner contents of the front layer
+          // from repainting when the backdrop toggle triggers a repaint on the
+          // LayoutBuilder.
+          return new RepaintBoundary(
+            child: new Column(
               mainAxisAlignment: MainAxisAlignment.center,
               mainAxisSize: MainAxisSize.min,
               crossAxisAlignment: CrossAxisAlignment.stretch,
@@ -139,6 +147,7 @@ class _CategoriesPage extends StatelessWidget {
                   }),
                 );
               }),
+            ),
           );
         },
       ),