Teach stack to size to the max of its non-positioned children

sky/sdk/lib/framework/rendering/stack.dart

Index: sky/sdk/lib/framework/rendering/stack.dart
diff --git a/sky/sdk/lib/framework/rendering/stack.dart b/sky/sdk/lib/framework/rendering/stack.dart
index c35020cf3f17596e1ea062b2abc9bb8df62f7cd5..53319ae7d315006fef7876550af577eb28d605f0 100644
--- a/sky/sdk/lib/framework/rendering/stack.dart
+++ b/sky/sdk/lib/framework/rendering/stack.dart
@@ -2,6 +2,8 @@
 // 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 'box.dart';
 import 'object.dart';
 
@@ -23,6 +25,8 @@ class StackParentData extends BoxParentData with ContainerParentDataMixin<Render
     super.merge(other);
   }
 
+  bool get isPositioned => top != null || right != null || bottom != null || left != null;
+
   String toString() => '${super.toString()}; top=$top; right=$right; bottom=$bottom, left=$left';
 }
 
@@ -40,68 +44,146 @@ class RenderStack extends RenderBox with ContainerRenderObjectMixin<RenderBox, S
       child.parentData = new StackParentData();
   }
 
+
   double getMinIntrinsicWidth(BoxConstraints constraints) {
-    return constraints.constrainWidth(double.INFINITY);
+    double width = constraints.minWidth;
+    RenderBox child = firstChild;
+    while (child != null) {
+      assert(child.parentData is StackParentData);
+      if (!child.parentData.isPositioned)
+        width = math.max(width, child.getMinIntrinsicWidth(constraints));
+      child = child.parentData.nextSibling;
+    }
+    assert(width == constraints.constrainWidth(width));
+    return width;
   }
 
   double getMaxIntrinsicWidth(BoxConstraints constraints) {
-    return constraints.constrainWidth(double.INFINITY);
+    bool hasNonPositionedChildren = false;
+    double width = constraints.minWidth;
+    RenderBox child = firstChild;
+    while (child != null) {
+      assert(child.parentData is StackParentData);
+      if (!child.parentData.isPositioned) {
+        hasNonPositionedChildren = true;
+        width = math.max(width, child.getMaxIntrinsicWidth(constraints));
+      }
+      child = child.parentData.nextSibling;
+    }
+    if (!hasNonPositionedChildren)
+      return constraints.constrainWidth(double.INFINITY);
+    assert(width == constraints.constrainWidth(width));
+    return width;
   }
 
   double getMinIntrinsicHeight(BoxConstraints constraints) {
-    return constraints.constrainHeight(double.INFINITY);
+    double height = constraints.minHeight;
+    RenderBox child = firstChild;
+    while (child != null) {
+      assert(child.parentData is StackParentData);
+      if (!child.parentData.isPositioned)
+        height = math.max(height, child.getMinIntrinsicHeight(constraints));
+      child = child.parentData.nextSibling;
+    }
+    assert(height == constraints.constrainHeight(height));
+    return height;
   }
 
   double getMaxIntrinsicHeight(BoxConstraints constraints) {
-    return constraints.constrainHeight(double.INFINITY);
+    bool hasNonPositionedChildren = false;
+    double height = constraints.minHeight;
+    RenderBox child = firstChild;
+    while (child != null) {
+      assert(child.parentData is StackParentData);
+      if (!child.parentData.isPositioned) {
+        hasNonPositionedChildren = true;
+        height = math.max(height, child.getMaxIntrinsicHeight(constraints));
+      }
+      child = child.parentData.nextSibling;
+    }
+    if (!hasNonPositionedChildren)
+      return constraints.constrainHeight(double.INFINITY);
+    assert(height == constraints.constrainHeight(height));
+    return height;
   }
 
   void performLayout() {
-    size = constraints.constrain(Size.infinite);
+    bool hasNonPositionedChildren = false;
+
+    double width = 0.0;
+    double height = 0.0;
+
+    RenderBox child = firstChild;
+    while (child != null) {
+      assert(child.parentData is StackParentData);
+      final StackParentData parentData = child.parentData;
+
+      if (!parentData.isPositioned) {
+        hasNonPositionedChildren = true;
+
+        child.layout(constraints, parentUsesSize: true);
+        parentData.position = Point.origin;
+
+        final Size childSize = child.size;
+        width = math.max(width, childSize.width);
+        height = math.max(height, childSize.height);
+      }
+
+      child = parentData.nextSibling;
+    }
+
+    if (hasNonPositionedChildren)
+      size = new Size(width, height);
+    else
+      size = constraints.constrain(Size.infinite);
+
     assert(size.width < double.INFINITY);
     assert(size.height < double.INFINITY);
+
     BoxConstraints innerConstraints = new BoxConstraints.loose(size);
 
-    RenderBox child = firstChild;
+    child = firstChild;
     while (child != null) {
       assert(child.parentData is StackParentData);
-      StackParentData parentData = child.parentData;
-
-      BoxConstraints childConstraints = innerConstraints;
-
-      if (parentData.left != null && parentData.right != null)
-        childConstraints = childConstraints.applyWidth(parentData.right - parentData.left);
-      else if (parentData.left != null)
-        childConstraints = childConstraints.applyMaxWidth(size.width - parentData.left);
-      else if (parentData.right != null)
-        childConstraints = childConstraints.applyMaxWidth(size.width - parentData.right);
-
-      if (parentData.top != null && parentData.bottom != null)
-        childConstraints = childConstraints.applyHeight(parentData.bottom - parentData.top);
-      else if (parentData.top != null)
-        childConstraints = childConstraints.applyMaxHeight(size.height - parentData.top);
-      else if (parentData.bottom != null)
-        childConstraints = childConstraints.applyMaxHeight(size.width - parentData.bottom);
-
-      child.layout(childConstraints);
-
-      double x = 0.0;
-      if (parentData.left != null)
-        x = parentData.left;
-      else if (parentData.right != null)
-        x = size.width - parentData.right - child.size.width;
-      assert(x >= 0.0 && x + child.size.width <= size.width);
-
-      double y = 0.0;
-      if (parentData.top != null)
-        y = parentData.top;
-      else if (parentData.bottom != null)
-        y = size.height - parentData.bottom - child.size.height;
-      assert(y >= 0.0 && y + child.size.height <= size.height);
-
-      parentData.position = new Point(x, y);
-
-      child = child.parentData.nextSibling;
+      final StackParentData parentData = child.parentData;
+
+      if (parentData.isPositioned) {
+        BoxConstraints childConstraints = innerConstraints;
+
+        if (parentData.left != null && parentData.right != null)
+          childConstraints = childConstraints.applyWidth(parentData.right - parentData.left);
+        else if (parentData.left != null)
+          childConstraints = childConstraints.applyMaxWidth(size.width - parentData.left);
+        else if (parentData.right != null)
+          childConstraints = childConstraints.applyMaxWidth(size.width - parentData.right);
+
+        if (parentData.top != null && parentData.bottom != null)
+          childConstraints = childConstraints.applyHeight(parentData.bottom - parentData.top);
+        else if (parentData.top != null)
+          childConstraints = childConstraints.applyMaxHeight(size.height - parentData.top);
+        else if (parentData.bottom != null)
+          childConstraints = childConstraints.applyMaxHeight(size.width - parentData.bottom);
+
+        child.layout(childConstraints);
+
+        double x = 0.0;
+        if (parentData.left != null)
+          x = parentData.left;
+        else if (parentData.right != null)
+          x = size.width - parentData.right - child.size.width;
+        assert(x >= 0.0 && x + child.size.width <= size.width);
+
+        double y = 0.0;
+        if (parentData.top != null)
+          y = parentData.top;
+        else if (parentData.bottom != null)
+          y = size.height - parentData.bottom - child.size.height;
+        assert(y >= 0.0 && y + child.size.height <= size.height);
+
+        parentData.position = new Point(x, y);
+      }
+
+      child = parentData.nextSibling;
     }
   }