Optimize repainting in Scaffold

Previously, we triggered a layout (and hence a repaint) when sliding the
draw because we gave the draw loose constraints. The drawer uses an
Align to move itself to the proper side of the screen, so it can have
tight constraints, which makes it a layout boundary.

Also, don't trigger a layout just because the Scaffold rebuilds. There
isn't any state in the scaffold custom layout, so it doesn't need to
repaint just because we created a new instance of the delegate.

Finally, add the debugging infrastructure I used to find these issues.

packages/flutter/lib/src/material/scaffold.dart

@@ -39,17 +39,17 @@ class _ScaffoldLayout extends MultiChildLayoutDelegate {
     // in this case the toolbar appears -after- the body in the stacking order,
     // so the toolbar's shadow is drawn on top of the body.
 
-    final BoxConstraints toolBarConstraints = looseConstraints.tightenWidth(size.width);
+    final BoxConstraints fullWidthConstraints = looseConstraints.tightenWidth(size.width);
     Size toolBarSize = Size.zero;
 
     if (isChild(_Child.toolBar)) {
-      toolBarSize = layoutChild(_Child.toolBar, toolBarConstraints);
+      toolBarSize = layoutChild(_Child.toolBar, fullWidthConstraints);
       positionChild(_Child.toolBar, Offset.zero);
     }
 
     if (isChild(_Child.body)) {
       final double bodyHeight = size.height - toolBarSize.height;
-      final BoxConstraints bodyConstraints = toolBarConstraints.tightenHeight(bodyHeight);
+      final BoxConstraints bodyConstraints = fullWidthConstraints.tightenHeight(bodyHeight);
       layoutChild(_Child.body, bodyConstraints);
       positionChild(_Child.body, new Offset(0.0, toolBarSize.height));
     }
@@ -63,7 +63,6 @@ class _ScaffoldLayout extends MultiChildLayoutDelegate {
     // non-zero height then it's inset from the parent's right and bottom edges
     // by _kFloatingActionButtonMargin.
 
-    final BoxConstraints fullWidthConstraints = looseConstraints.tightenWidth(size.width);
     Size bottomSheetSize = Size.zero;
     Size snackBarSize = Size.zero;
 
@@ -89,10 +88,12 @@ class _ScaffoldLayout extends MultiChildLayoutDelegate {
     }
 
     if (isChild(_Child.drawer)) {
-      layoutChild(_Child.drawer, looseConstraints);
+      layoutChild(_Child.drawer, new BoxConstraints.tight(size));
       positionChild(_Child.drawer, Offset.zero);
     }
   }
+
+  bool shouldRelayout(MultiChildLayoutDelegate oldDelegate) => false;
 }
 
 class Scaffold extends StatefulComponent {

packages/flutter/lib/src/rendering/custom_layout.dart

@@ -92,7 +92,7 @@ abstract class MultiChildLayoutDelegate {
   }
 
   /// Override this method to return true when the children need to be laid out.
-  bool shouldRelayout(MultiChildLayoutDelegate oldDelegate) => true;
+  bool shouldRelayout(MultiChildLayoutDelegate oldDelegate);
 
   /// Layout and position all children given this widget's size and the specified
   /// constraints. This method must apply layoutChild() to each child. It should

packages/flutter/lib/src/rendering/debug.dart

@@ -52,6 +52,12 @@ HSVColor debugCurrentRepaintColor = const HSVColor.fromAHSV(0.4, 60.0, 1.0, 1.0)
 /// The amount to increment the hue of the current repaint color.
 double debugRepaintRainboxHueIncrement = 2.0;
 
+/// Log the call stacks that mark render objects as needing paint.
+bool debugPrintMarkNeedsPaintStacks = false;
+
+/// Log the call stacks that mark render objects as needing layout.
+bool debugPrintMarkNeedsLayoutStacks = false;
+
 List<String> debugDescribeTransform(Matrix4 transform) {
   List<String> matrix = transform.toString().split('\n').map((String s) => '  $s').toList();
   matrix.removeLast();

packages/flutter/lib/src/rendering/object.dart

@@ -8,6 +8,7 @@ import 'dart:ui' as ui;
 
 import 'package:flutter/gestures.dart';
 import 'package:flutter/scheduler.dart';
+import 'package:flutter/services.dart';
 import 'package:vector_math/vector_math_64.dart';
 
 import 'debug.dart';
@@ -556,6 +557,11 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
       }
       assert(parent == this.parent);
     } else {
+      assert(() {
+        if (debugPrintMarkNeedsLayoutStacks)
+          debugPrintStack();
+        return true;
+      });
       _nodesNeedingLayout.add(this);
       Scheduler.instance.ensureVisualUpdate();
     }
@@ -950,6 +956,11 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
       return;
     _needsPaint = true;
     if (hasLayer) {
+      assert(() {
+        if (debugPrintMarkNeedsPaintStacks)
+          debugPrintStack();
+        return true;
+      });
       // If we always have our own layer, then we can just repaint
       // ourselves without involving any other nodes.
       assert(_layer != null);

packages/flutter/lib/src/services/print.dart

@@ -44,3 +44,11 @@ void _debugPrintTask() {
     _debugPrintStopwatch.start();
   }
 }
+
+void debugPrintStack() {
+  try {
+    throw new Exception();
+  } catch (e, stack) {
+    debugPrint(stack.toString());
+  }
+}