Remove the inner SizeObserver from ScrollableWidgetList.

Adds a HomogeneousViewport class that works like MixedViewport but
handles only children that have all the same height.

Converts ScrollableWidgetList to use that, so that we don't waste a
frame looking at the size of the contents each time we change size.

This allows a number of seemingly pointless double-pumps in the tests
to be removed.

Other changes that were necessary to support the above:

 - RenderBlock now supports minExtent (think 'min-height' in CSS)
 - RenderBlock now supports itemExtent (forces the height of each
   child to be the same, so that the itemExtent passed to the fixed-
   height scrollables are all authoritative instead of a source of
   bugs when they don't match)
 - RenderBlockViewport now supports horizontal scrolling
 - improved the style of the isInfinite assert in box.dart
 - fixed the position of a comment in mixed_viewport.dart
 - added a test
 - made the logic for how many items to show be more precise

sky/packages/sky/lib/rendering/block.dart

@@ -23,8 +23,10 @@ abstract class RenderBlockBase extends RenderBox with ContainerRenderObjectMixin
 
   RenderBlockBase({
     List<RenderBox> children,
-    BlockDirection direction: BlockDirection.vertical
-  }) : _direction = direction {
+    BlockDirection direction: BlockDirection.vertical,
+    double itemExtent,
+    double minExtent: 0.0
+  }) : _direction = direction, _itemExtent = itemExtent, _minExtent = minExtent {
     addAll(children);
   }
 
@@ -42,22 +44,42 @@ abstract class RenderBlockBase extends RenderBox with ContainerRenderObjectMixin
     }
   }
 
+  double _itemExtent;
+  double get itemExtent => _itemExtent;
+  void set itemExtent(double value) {
+    if (value != _itemExtent) {
+      _itemExtent = value;
+      markNeedsLayout();
+    }
+  }
+
+  double _minExtent;
+  double get minExtent => _minExtent;
+  void set minExtent(double value) {
+    if (value != _minExtent) {
+      _minExtent = value;
+      markNeedsLayout();
+    }
+  }
+
   bool get isVertical => _direction == BlockDirection.vertical;
 
   BoxConstraints _getInnerConstraints(BoxConstraints constraints) {
     if (isVertical)
-      return new BoxConstraints.tightFor(width: constraints.constrainWidth(constraints.maxWidth));
-    return new BoxConstraints.tightFor(height: constraints.constrainHeight(constraints.maxHeight));
+      return new BoxConstraints.tightFor(width: constraints.constrainWidth(constraints.maxWidth),
+                                         height: itemExtent);
+    return new BoxConstraints.tightFor(height: constraints.constrainHeight(constraints.maxHeight),
+                                       width: itemExtent);
   }
 
   double get _mainAxisExtent {
     RenderBox child = lastChild;
     if (child == null)
-      return 0.0;
+      return minExtent;
     BoxParentData parentData = child.parentData;
     return isVertical ?
-        parentData.position.y + child.size.height :
-        parentData.position.x + child.size.width;
+        math.max(minExtent, parentData.position.y + child.size.height) :
+        math.max(minExtent, parentData.position.x + child.size.width);
   }
 
   void performLayout() {
@@ -84,8 +106,10 @@ class RenderBlock extends RenderBlockBase {
 
   RenderBlock({
     List<RenderBox> children,
-    BlockDirection direction: BlockDirection.vertical
-  }) : super(children: children, direction: direction);
+    BlockDirection direction: BlockDirection.vertical,
+    double itemExtent,
+    double minExtent: 0.0
+  }) : super(children: children, direction: direction, itemExtent: itemExtent, minExtent: minExtent);
 
   double _getIntrinsicCrossAxis(BoxConstraints constraints, _ChildSizingFunction childSize) {
     double extent = 0.0;
@@ -116,7 +140,7 @@ class RenderBlock extends RenderBlockBase {
       assert(child.parentData is BlockParentData);
       child = child.parentData.nextSibling;
     }
-    return extent;
+    return math.max(extent, minExtent);
   }
 
   double getMinIntrinsicWidth(BoxConstraints constraints) {
@@ -185,15 +209,17 @@ class RenderBlockViewport extends RenderBlockBase {
     DimensionCallback totalExtentCallback,
     DimensionCallback maxCrossAxisDimensionCallback,
     DimensionCallback minCrossAxisDimensionCallback,
+    BlockDirection direction: BlockDirection.vertical,
+    double itemExtent,
+    double minExtent: 0.0,
     double startOffset: 0.0,
-    List<RenderBox> children,
-    BlockDirection direction: BlockDirection.vertical
+    List<RenderBox> children
   }) : _callback = callback,
        _totalExtentCallback = totalExtentCallback,
        _maxCrossAxisDimensionCallback = maxCrossAxisDimensionCallback,
        _minCrossAxisDimensionCallback = minCrossAxisDimensionCallback,
        _startOffset = startOffset,
-       super(children: children, direction: direction);
+       super(children: children, direction: direction, itemExtent: itemExtent, minExtent: minExtent);
 
   bool _inCallback = false;
 
@@ -250,11 +276,10 @@ class RenderBlockViewport extends RenderBlockBase {
   double _startOffset;
   double get startOffset => _startOffset;
   void set startOffset(double value) {
-    if (value == _startOffset)
-      return;
-    _startOffset = value;
-    if (!_inCallback)
+    if (value != _startOffset) {
+      _startOffset = value;
       markNeedsPaint();
+    }
   }
 
   double _getIntrinsicDimension(BoxConstraints constraints, DimensionCallback intrinsicCallback, _Constrainer constrainer) {
@@ -283,13 +308,13 @@ class RenderBlockViewport extends RenderBlockBase {
   double getMinIntrinsicWidth(BoxConstraints constraints) {
     if (isVertical)
       return _getIntrinsicDimension(constraints, minCrossAxisDimensionCallback, constraints.constrainWidth);
-    return constraints.constrainWidth(0.0);
+    return constraints.constrainWidth(minExtent);
   }
 
   double getMaxIntrinsicWidth(BoxConstraints constraints) {
     if (isVertical)
       return _getIntrinsicDimension(constraints, maxCrossAxisDimensionCallback, constraints.constrainWidth);
-    return _getIntrinsicDimension(constraints, totalExtentCallback, constraints.constrainWidth);
+    return _getIntrinsicDimension(constraints, totalExtentCallback, new BoxConstraints(minWidth: minExtent).apply(constraints).constrainWidth);
   }
 
   double getMinIntrinsicHeight(BoxConstraints constraints) {
@@ -301,7 +326,7 @@ class RenderBlockViewport extends RenderBlockBase {
   double getMaxIntrinsicHeight(BoxConstraints constraints) {
     if (!isVertical)
       return _getIntrinsicDimension(constraints, maxCrossAxisDimensionCallback, constraints.constrainHeight);
-    return _getIntrinsicDimension(constraints, totalExtentCallback, constraints.constrainHeight);
+    return _getIntrinsicDimension(constraints, totalExtentCallback, new BoxConstraints(minHeight: minExtent).apply(constraints).constrainHeight);
   }
 
   // We don't override computeDistanceToActualBaseline(), because we
@@ -325,7 +350,10 @@ class RenderBlockViewport extends RenderBlockBase {
   void paint(PaintingContext context, Offset offset) {
     context.canvas.save();
     context.canvas.clipRect(offset & size);
-    defaultPaint(context, offset.translate(0.0, startOffset));
+    if (isVertical)
+      defaultPaint(context, offset.translate(0.0, startOffset));
+    else
+      defaultPaint(context, offset.translate(startOffset, 0.0));
     context.canvas.restore();
   }
 

sky/packages/sky/lib/rendering/box.dart

@@ -319,7 +319,10 @@ abstract class RenderBox extends RenderObject {
   bool debugDoesMeetConstraints() {
     assert(constraints != null);
     assert(_size != null);
-    assert(!_size.isInfinite && 'See https://github.com/domokit/sky_engine/blob/master/sky/packages/sky/lib/widgets/sizing.md#user-content-unbounded-constraints' is String);
+    assert(() {
+      'See https://github.com/domokit/sky_engine/blob/master/sky/packages/sky/lib/widgets/sizing.md#user-content-unbounded-constraints';
+      return !_size.isInfinite;
+    });
     bool result = constraints.contains(_size);
     if (!result)
       print("${this.runtimeType} does not meet its constraints. Constraints: $constraints, size: $_size");

sky/packages/sky/lib/widgets.dart

@@ -24,6 +24,7 @@ export 'package:sky/widgets/floating_action_button.dart';
 export 'package:sky/widgets/focus.dart';
 export 'package:sky/widgets/framework.dart';
 export 'package:sky/widgets/gesture_detector.dart';
+export 'package:sky/widgets/homogeneous_viewport.dart';
 export 'package:sky/widgets/icon.dart';
 export 'package:sky/widgets/icon_button.dart';
 export 'package:sky/widgets/ink_well.dart';

sky/packages/sky/lib/widgets/homogeneous_viewport.dart

@@ -0,0 +1,179 @@
+// Copyright 2015 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.
+
+import 'dart:math' as math;
+
+import 'package:sky/rendering/block.dart';
+import 'package:sky/rendering/box.dart';
+import 'package:sky/rendering/object.dart';
+import 'package:sky/widgets/framework.dart';
+import 'package:sky/widgets/basic.dart';
+
+typedef List<Widget> ListBuilder(int startIndex, int count);
+
+class HomogeneousViewport extends RenderObjectWrapper {
+  HomogeneousViewport({
+    Key key,
+    this.builder,
+    this.itemExtent, // required
+    this.itemCount, // optional, but you cannot shrink-wrap this class or otherwise use its intrinsic dimensions if you don't specify it
+    this.direction: ScrollDirection.vertical,
+    this.startOffset: 0.0
+  }) : super(key: key) {
+    assert(itemExtent != null);
+  }
+
+  ListBuilder builder;
+  double itemExtent;
+  int itemCount;
+  ScrollDirection direction;
+  double startOffset;
+
+  bool _layoutDirty = true;
+  List<Widget> _children;
+
+  RenderBlockViewport get renderObject => super.renderObject;
+
+  RenderBlockViewport createNode() {
+    // we don't pass constructor arguments to the RenderBlockViewport() because until
+    // we know our children, the constructor arguments we could give have no effect
+    RenderBlockViewport result = new RenderBlockViewport();
+    result.callback = layout;
+    result.totalExtentCallback = getTotalExtent;
+    result.minCrossAxisDimensionCallback = getMinCrossAxisDimension;
+    result.maxCrossAxisDimensionCallback = getMaxCrossAxisDimension;
+    return result;
+  }
+
+  void remove() {
+    renderObject.callback = null;
+    renderObject.totalExtentCallback = null;
+    renderObject.minCrossAxisDimensionCallback = null;
+    renderObject.maxCrossAxisDimensionCallback = null;
+    super.remove();
+    _children.clear();
+    _layoutDirty = true;
+  }
+
+  void walkChildren(WidgetTreeWalker walker) {
+    if (_children == null) return;
+    for (Widget child in _children)
+      walker(child);
+  }
+
+  void insertChildRenderObject(RenderObjectWrapper child, Widget slot) {
+    RenderObject nextSibling = slot != null ? slot.renderObject : null;
+    renderObject.add(child.renderObject, before: nextSibling);
+  }
+
+  void detachChildRenderObject(RenderObjectWrapper child) {
+    renderObject.remove(child.renderObject);
+  }
+
+  bool retainStatefulNodeIfPossible(HomogeneousViewport newNode) {
+    retainStatefulRenderObjectWrapper(newNode);
+    if (startOffset != newNode.startOffset) {
+      _layoutDirty = true;
+      startOffset = newNode.startOffset;
+    }
+    if (itemCount != newNode.itemCount) {
+      _layoutDirty = true;
+      itemCount = newNode.itemCount;
+    }
+    if (itemExtent != newNode.itemExtent) {
+      _layoutDirty = true;
+      itemExtent = newNode.itemExtent;
+    }
+    if (direction != newNode.direction) {
+      _layoutDirty = true;
+      direction = newNode.direction;
+    }
+    if (builder != newNode.builder) {
+      _layoutDirty = true;
+      builder = newNode.builder;
+    }
+    return true;
+  }
+
+  // This is called during the regular component build
+  void syncRenderObject(HomogeneousViewport old) {
+    super.syncRenderObject(old);
+    if (_layoutDirty) {
+      renderObject.markNeedsLayout();
+    } else {
+      assert(old != null); // if old was null, we'd be new, and therefore _layoutDirty would be true
+      _updateChildren();
+    }
+  }
+
+  int _layoutFirstIndex;
+  int _layoutItemCount;
+
+  void layout(BoxConstraints constraints) {
+    LayoutCallbackBuilderHandle handle = enterLayoutCallbackBuilder();
+    try {
+      double mainAxisExtent = direction == ScrollDirection.vertical ? constraints.maxHeight : constraints.maxWidth;
+      double offset;
+      if (startOffset <= 0.0) {
+        _layoutFirstIndex = 0;
+        offset = -startOffset;
+      } else {
+        _layoutFirstIndex = startOffset ~/ itemExtent;
+        offset = -(startOffset % itemExtent);
+      }
+      if (mainAxisExtent < double.INFINITY) {
+        _layoutItemCount = ((mainAxisExtent - offset) / itemExtent).ceil();
+        if (itemCount != null)
+          _layoutItemCount = math.min(_layoutItemCount, itemCount - _layoutFirstIndex);
+      } else {
+        assert(() {
+          'This HomogeneousViewport has no specified number of items (meaning it has infinite items), ' +
+          'and has been placed in an unconstrained environment where all items can be rendered. ' +
+          'It is most likely that you have placed your HomogeneousViewport (which is an internal ' +
+          'component of several scrollable widgets) inside either another scrolling box, a flexible ' +
+          'box (Row, Column), or a Stack, without giving it a specific size.';
+          return itemCount != null;
+        });
+        _layoutItemCount = itemCount - _layoutFirstIndex;
+      }
+      _layoutItemCount = math.max(0, _layoutItemCount);
+      _updateChildren();
+      // Update the renderObject configuration
+      renderObject.direction = direction == ScrollDirection.vertical ? BlockDirection.vertical : BlockDirection.horizontal;
+      renderObject.itemExtent = itemExtent;
+      renderObject.minExtent = getTotalExtent(null);
+      renderObject.startOffset = offset;
+    } finally {
+      exitLayoutCallbackBuilder(handle);
+    }
+  }
+
+  void _updateChildren() {
+    assert(_layoutFirstIndex != null);
+    assert(_layoutItemCount != null);
+    List<Widget> newChildren;
+    if (_layoutItemCount > 0)
+      newChildren = builder(_layoutFirstIndex, _layoutItemCount);
+    else
+      newChildren = <Widget>[];
+    syncChildren(newChildren, _children == null ? <Widget>[] : _children);
+    _children = newChildren;
+  }
+
+  double getTotalExtent(BoxConstraints constraints) {
+    // constraints is null when called by layout() above
+    return itemCount != null ? itemCount * itemExtent : double.INFINITY;
+  }
+
+  double getMinCrossAxisDimension(BoxConstraints constraints) {
+    return 0.0;
+  }
+
+  double getMaxCrossAxisDimension(BoxConstraints constraints) {
+    if (direction == ScrollDirection.vertical)
+      return constraints.maxWidth;
+    return constraints.maxHeight;
+  }
+
+}

sky/packages/sky/lib/widgets/mixed_viewport.dart

@@ -91,11 +91,11 @@ class MixedViewport extends RenderObjectWrapper {
 
   Map<_Key, Widget> _childrenByKey = new Map<_Key, Widget>();
 
-  // we don't pass the direction or offset to the render object when we create it, because
-  // the render object is empty so it will not matter
   RenderBlockViewport get renderObject => super.renderObject;
 
   RenderBlockViewport createNode() {
+    // we don't pass the direction or offset to the render object when we
+    // create it, because the render object is empty so it will not matter
     RenderBlockViewport result = new RenderBlockViewport();
     result.callback = layout;
     result.totalExtentCallback = _noIntrinsicDimensions;

sky/packages/sky/lib/widgets/scrollable.dart

@@ -18,6 +18,7 @@ import 'package:sky/rendering/viewport.dart';
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/framework.dart';
 import 'package:sky/widgets/gesture_detector.dart';
+import 'package:sky/widgets/homogeneous_viewport.dart';
 import 'package:sky/widgets/mixed_viewport.dart';
 
 export 'package:sky/widgets/mixed_viewport.dart' show MixedViewportLayoutState;
@@ -421,17 +422,11 @@ abstract class ScrollableWidgetList extends Scrollable {
     double contentExtent = itemExtent * itemCount;
     if (padding != null)
       contentExtent += _leadingPadding + _trailingPadding;
-
     scrollTo(scrollBehavior.updateExtents(
       contentExtent: contentExtent,
       containerExtent: _containerExtent,
-      scrollOffset: scrollOffset));
-  }
-
-  Offset _toOffset(double value) {
-    return scrollDirection == ScrollDirection.vertical
-      ? new Offset(0.0, value)
-      : new Offset(value, 0.0);
+      scrollOffset: scrollOffset
+    ));
   }
 
   Widget buildContent() {
@@ -440,48 +435,27 @@ abstract class ScrollableWidgetList extends Scrollable {
       _updateScrollBehavior();
     }
 
-    double paddedScrollOffset = scrollOffset - _leadingPadding;
-    int itemShowIndex = 0;
-    int itemShowCount = 0;
-    Offset viewportOffset = Offset.zero;
-
-    if (_containerExtent != null && _containerExtent > 0.0 && itemCount > 0) {
-      if (paddedScrollOffset < scrollBehavior.minScrollOffset) {
-        // Underscroll
-        double visibleExtent = _containerExtent + paddedScrollOffset;
-        itemShowCount = (visibleExtent / itemExtent).ceil();
-        viewportOffset = _toOffset(paddedScrollOffset);
-      } else {
-        itemShowCount = (_containerExtent / itemExtent).ceil() + 1;
-        itemShowIndex = (paddedScrollOffset / itemExtent).floor();
-        viewportOffset = _toOffset(paddedScrollOffset - itemShowIndex * itemExtent);
-        itemShowIndex %= itemCount; // Wrap index for when itemWrap is true.
-      }
-    }
-
-    List<Widget> items = buildItems(itemShowIndex, itemShowCount);
-    assert(items.every((item) => item.key != null));
-
-    BlockDirection blockDirection = scrollDirection == ScrollDirection.vertical
-      ? BlockDirection.vertical
-      : BlockDirection.horizontal;
-
-    // TODO(ianh): Refactor this so that it does the building in the
-    // same frame as the size observing, similar to MixedViewport, but
-    // keeping the fixed-height optimisations.
     return new SizeObserver(
       callback: _handleSizeChanged,
-      child: new Viewport(
-        scrollDirection: scrollDirection,
-        scrollOffset: viewportOffset,
-        child: new Container(
-          padding: _crossAxisPadding,
-          child: new BlockBody(items, direction: blockDirection)
+      child: new Container(
+        padding: _crossAxisPadding,
+        child: new HomogeneousViewport(
+          builder: _buildItems,
+          itemExtent: itemExtent,
+          itemCount: itemCount,
+          direction: scrollDirection,
+          startOffset: scrollOffset - _leadingPadding
         )
       )
     );
   }
 
+  List<Widget> _buildItems(int start, int count) {
+    List<Widget> result = buildItems(start, count);
+    assert(result.every((item) => item.key != null));
+    return result;
+  }
+
   List<Widget> buildItems(int start, int count);
 
 }

sky/unit/test/widget/date_picker_test.dart

@@ -22,19 +22,13 @@ void main() {
       ]);
     }
 
-    tester.pumpFrame(builder);
-    // TODO(abarth): We shouldn't need to pump a second frame here.
     tester.pumpFrame(builder);
 
     expect(currentValue, isNull);
     tester.tap(tester.findText('2015'));
     tester.pumpFrame(builder);
-    // TODO(jackson): We shouldn't need to pump a second frame here.
-    tester.pumpFrame(builder);
     tester.tap(tester.findText('2014'));
     tester.pumpFrame(builder);
-    // TODO(jackson): We shouldn't need to pump a second frame here.
-    tester.pumpFrame(builder);
     expect(currentValue, equals(new DateTime(2014, 6, 9)));
     tester.tap(tester.findText('30'));
     expect(currentValue, equals(new DateTime(2013, 1, 30)));

sky/unit/test/widget/homogeneous_viewport_test.dart

@@ -0,0 +1,170 @@
+import 'package:sky/widgets.dart';
+import 'package:test/test.dart';
+
+import 'widget_tester.dart';
+
+class TestComponent extends StatefulComponent {
+  TestComponent(this.viewport);
+  HomogeneousViewport viewport;
+  void syncConstructorArguments(TestComponent source) {
+    viewport = source.viewport;
+  }
+  bool _flag = true;
+  void go(bool flag) {
+    setState(() {
+      _flag = flag;
+    });
+  }
+  Widget build() {
+    return _flag ? viewport : new Text('Not Today');
+  }
+}
+
+void main() {
+  test('HomogeneousViewport mount/dismount smoke test', () {
+    WidgetTester tester = new WidgetTester();
+
+    List<int> callbackTracker = <int>[];
+
+    // the root view is 800x600 in the test environment
+    // so if our widget is 100 pixels tall, it should fit exactly 6 times.
+
+    TestComponent testComponent;
+    Widget builder() {
+      testComponent = new TestComponent(new HomogeneousViewport(
+        builder: (int start, int count) {
+          List<Widget> result = <Widget>[];
+          for (int index = start; index < start + count; index += 1) {
+            callbackTracker.add(index);
+            result.add(new Container(
+              key: new ValueKey<int>(index),
+              height: 100.0,
+              child: new Text("$index")
+            ));
+          }
+          return result;
+        },
+        startOffset: 0.0,
+        itemExtent: 100.0
+      ));
+      return testComponent;
+    }
+
+    tester.pumpFrame(builder);
+
+    expect(callbackTracker, equals([0, 1, 2, 3, 4, 5]));
+
+    callbackTracker.clear();
+    testComponent.go(false);
+    tester.pumpFrameWithoutChange();
+
+    expect(callbackTracker, equals([]));
+
+    callbackTracker.clear();
+    testComponent.go(true);
+    tester.pumpFrameWithoutChange();
+
+    expect(callbackTracker, equals([0, 1, 2, 3, 4, 5]));
+  });
+
+  test('HomogeneousViewport vertical', () {
+    WidgetTester tester = new WidgetTester();
+
+    List<int> callbackTracker = <int>[];
+
+    // the root view is 800x600 in the test environment
+    // so if our widget is 200 pixels tall, it should fit exactly 3 times.
+    // but if we are offset by 300 pixels, there will be 4, numbered 1-4.
+
+    double offset = 300.0;
+
+    ListBuilder itemBuilder = (int start, int count) {
+      List<Widget> result = <Widget>[];
+      for (int index = start; index < start + count; index += 1) {
+        callbackTracker.add(index);
+        result.add(new Container(
+          key: new ValueKey<int>(index),
+          width: 500.0, // this should be ignored
+          height: 400.0, // should be overridden by itemExtent
+          child: new Text("$index")
+        ));
+      }
+      return result;
+    };
+
+    TestComponent testComponent;
+    Widget builder() {
+      testComponent = new TestComponent(new HomogeneousViewport(
+        builder: itemBuilder,
+        startOffset: offset,
+        itemExtent: 200.0
+      ));
+      return testComponent;
+    }
+
+    tester.pumpFrame(builder);
+
+    expect(callbackTracker, equals([1, 2, 3, 4]));
+
+    callbackTracker.clear();
+
+    offset = 400.0; // now only 3 should fit, numbered 2-4.
+
+    tester.pumpFrame(builder);
+
+    expect(callbackTracker, equals([2, 3, 4]));
+
+    callbackTracker.clear();
+  });
+
+  test('HomogeneousViewport horizontal', () {
+    WidgetTester tester = new WidgetTester();
+
+    List<int> callbackTracker = <int>[];
+
+    // the root view is 800x600 in the test environment
+    // so if our widget is 200 pixels wide, it should fit exactly 4 times.
+    // but if we are offset by 300 pixels, there will be 5, numbered 1-5.
+
+    double offset = 300.0;
+
+    ListBuilder itemBuilder = (int start, int count) {
+      List<Widget> result = <Widget>[];
+      for (int index = start; index < start + count; index += 1) {
+        callbackTracker.add(index);
+        result.add(new Container(
+          key: new ValueKey<int>(index),
+          width: 400.0, // this should be overridden by itemExtent
+          height: 500.0, // this should be ignored
+          child: new Text("$index")
+        ));
+      }
+      return result;
+    };
+
+    TestComponent testComponent;
+    Widget builder() {
+      testComponent = new TestComponent(new HomogeneousViewport(
+        builder: itemBuilder,
+        startOffset: offset,
+        itemExtent: 200.0,
+        direction: ScrollDirection.horizontal
+      ));
+      return testComponent;
+    }
+
+    tester.pumpFrame(builder);
+
+    expect(callbackTracker, equals([1, 2, 3, 4, 5]));
+
+    callbackTracker.clear();
+
+    offset = 400.0; // now only 4 should fit, numbered 2-5.
+
+    tester.pumpFrame(builder);
+
+    expect(callbackTracker, equals([2, 3, 4, 5]));
+
+    callbackTracker.clear();
+  });
+}

sky/unit/test/widget/pageable_list_test.dart

@@ -37,8 +37,6 @@ void main() {
       );
     }
 
-    tester.pumpFrame(builder);
-    // TODO(abarth): We shouldn't need to pump a second frame here.
     tester.pumpFrame(builder);
 
     expect(currentPage, isNull);