Make the drawer, popup menus, dialogs, and settings page scrollable.

sky/sdk/lib/rendering/box.dart

Index: sky/sdk/lib/rendering/box.dart
diff --git a/sky/sdk/lib/rendering/box.dart b/sky/sdk/lib/rendering/box.dart
index 460956bacb482e35f7f27a5266c12626bf64a0ad..9db83b9c32c33ffb6343587032e00f865744b9c4 100644
--- a/sky/sdk/lib/rendering/box.dart
+++ b/sky/sdk/lib/rendering/box.dart
@@ -1014,6 +1014,103 @@ class RenderBaseline extends RenderShiftedBox {
   String debugDescribeSettings(String prefix) => '${super.debugDescribeSettings(prefix)}${prefix}baseline: ${baseline}\nbaselineType: ${baselineType}';
 }
 
+enum ViewportScrollDirection { horizontal, vertical, both }
+
+class RenderViewport extends RenderBox with RenderObjectWithChildMixin<RenderBox> {
+
+  RenderViewport({
+    RenderBox child,
+    Offset scrollOffset,
+    ViewportScrollDirection direction: ViewportScrollDirection.vertical
+  }) : _scrollOffset = scrollOffset,
+       _scrollDirection = direction {
+    this.child = child;
+  }
+
+  Offset _scrollOffset;
+  Offset get scrollOffset => _scrollOffset;
+  void set scrollOffset(Offset value) {
+    if (value == _scrollOffset)
+      return;
+    _scrollOffset = value;

[abarth-chromium, 2015/07/09 23:39:34]

Probably should assert that _scrollOffset respects |direction|.  (Also in the
constructor)

+    markNeedsPaint();
+  }
+
+  ViewportScrollDirection _scrollDirection;
+  ViewportScrollDirection get scrollDirection => _scrollDirection;
+  void set scrollDirection(ViewportScrollDirection value) {
+    if (value == _scrollDirection)
+      return;
+    _scrollDirection = value;
+    markNeedsLayout();
+  }
+
+  double getMinIntrinsicWidth(BoxConstraints constraints) {
+    if (child != null && scrollDirection != ViewportScrollDirection.vertical)
+      return child.getMinIntrinsicWidth(constraints);
+    return constraints.constrainWidth();
+  }
+
+  double getMaxIntrinsicWidth(BoxConstraints constraints) {
+    if (child != null && scrollDirection != ViewportScrollDirection.vertical)
+      return child.getMaxIntrinsicWidth(constraints);
+    return constraints.constrainWidth();
+  }
+
+  double getMinIntrinsicHeight(BoxConstraints constraints) {
+    if (child != null && scrollDirection != ViewportScrollDirection.horizontal)
+      return child.getMinIntrinsicHeight(constraints);
+    return constraints.constrainHeight();
+  }
+
+  double getMaxIntrinsicHeight(BoxConstraints constraints) {
+    if (child != null && scrollDirection != ViewportScrollDirection.horizontal)
+      return child.getMaxIntrinsicHeight(constraints);
+    return constraints.constrainHeight();
+  }
+
+  // We don't override computeDistanceToActualBaseline(), because we
+  // want the default behaviour (returning null). Otherwise, as you
+  // scroll the RenderBlockViewport, it would shift in its parent if
+  // the parent was baseline-aligned, which makes no sense.
+
+  void performLayout() {
+    if (child != null) {
+      BoxConstraints innerConstraints;
+      switch (scrollDirection) {
+        case ViewportScrollDirection.both:
+          innerConstraints = new BoxConstraints(); break;

[abarth-chromium, 2015/07/09 23:39:35]

One statement per line please.

+        case ViewportScrollDirection.horizontal:
+          innerConstraints = constraints.heightConstraints(); break;
+        case ViewportScrollDirection.vertical:
+          innerConstraints = constraints.widthConstraints(); break;
+      }
+      child.layout(innerConstraints, parentUsesSize: true);
+      size = constraints.constrain(child.size);
+      assert(child.parentData is BoxParentData);
+      child.parentData.position = Point.origin;
+    }
+  }

[abarth-chromium, 2015/07/09 23:39:35]

Don't we need to size ourselves to something even if we have no child? 
RenderProxyBox calls performResize() to do that (which makes itself as small as
possible).

+
+  void paint(PaintingCanvas canvas, Offset offset) {
+    canvas.save();
+    canvas.clipRect(offset & size);

[abarth-chromium, 2015/07/09 23:39:35]

We should only clip if the child is actually bigger than we are.  (Also, we
shouldn't make an empty clip if we have no child)

+    if (child != null)
+      canvas.paintChild(child, (offset - scrollOffset).toPoint());
+    canvas.restore();
+  }
+
+  void hitTestChildren(HitTestResult result, { Point position }) {
+    if (child != null) {
+      assert(child.parentData is BoxParentData);
+      Rect childBounds = child.parentData.position & child.size;
+      if (childBounds.contains(position + -scrollOffset))
+        child.hitTest(result, position: position + scrollOffset);
+    }
+  }
+
+}
+
 class RenderImage extends RenderBox {
 
   RenderImage(sky.Image image, Size requestedSize)