Make Flex shrink-wrap when unconstrained.

packages/flutter/lib/rendering/flex.dart

@@ -289,38 +289,49 @@ class RenderFlex extends RenderBox with ContainerRenderObjectMixin<RenderBox, Fl
   }
 
   void performLayout() {
-    // Based on http://www.w3.org/TR/css-flexbox-1/ Section 9.7 Resolving Flexible Lengths
+    // Originally based on http://www.w3.org/TR/css-flexbox-1/ Section 9.7 Resolving Flexible Lengths
-    // Steps 1-3. Determine used flex factor, size inflexible items, calculate free space
+
+    // Determine used flex factor, size inflexible items, calculate free space.
     int totalFlex = 0;
     int totalChildren = 0;
     assert(constraints != null);
     final double mainSize = (_direction == FlexDirection.horizontal) ? constraints.maxWidth : constraints.maxHeight;
-    double crossSize = 0.0;  // This will be determined after laying out the children
+    final bool canFlex = mainSize < double.INFINITY;
-    double freeSpace = mainSize;
+    double crossSize = 0.0;  // This is determined as we lay out the children
+    double freeSpace = canFlex ? mainSize : 0.0;
     RenderBox child = firstChild;
     while (child != null) {
       assert(child.parentData is FlexBoxParentData);
       totalChildren++;
       int flex = _getFlex(child);
       if (flex > 0) {
+        // Flexible children can only be used when the RenderFlex box's container has a finite size.
+        // When the container is infinite, for example if you are in a scrollable viewport, then 
+        // it wouldn't make any sense to have a flexible child.
+        assert(canFlex && 'See https://github.com/domokit/sky_engine/blob/master/sky/packages/sky/lib/widgets/flex.md' is String);
         totalFlex += child.parentData.flex;
       } else {
         BoxConstraints innerConstraints;
         if (alignItems == FlexAlignItems.stretch) {
           switch (_direction) {
             case FlexDirection.horizontal:
-              innerConstraints = new BoxConstraints(maxWidth: constraints.maxWidth,
+              innerConstraints = new BoxConstraints(minHeight: constraints.minHeight,
-                                                    minHeight: constraints.minHeight,
                                                     maxHeight: constraints.maxHeight);
               break;
             case FlexDirection.vertical:
               innerConstraints = new BoxConstraints(minWidth: constraints.minWidth,
-                                                    maxWidth: constraints.maxWidth,
+                                                    maxWidth: constraints.maxWidth);
-                                                    maxHeight: constraints.maxHeight);
               break;
           }
         } else {
-          innerConstraints = constraints.loosen();
+          switch (_direction) {
+            case FlexDirection.horizontal:
+              innerConstraints = new BoxConstraints(maxHeight: constraints.maxHeight);
+              break;
+            case FlexDirection.vertical:
+              innerConstraints = new BoxConstraints(maxWidth: constraints.maxWidth);
+              break;
+          }
         }
         child.layout(innerConstraints, parentUsesSize: true);
         freeSpace -= _getMainSize(child);
@@ -331,10 +342,11 @@ class RenderFlex extends RenderBox with ContainerRenderObjectMixin<RenderBox, Fl
     _overflow = math.max(0.0, -freeSpace);
     freeSpace = math.max(0.0, freeSpace);
 
-    // Steps 4-5. Distribute remaining space to flexible children.
+    // Distribute remaining space to flexible children, and determine baseline.
-    double spacePerFlex = totalFlex > 0 ? (freeSpace / totalFlex) : 0.0;
-    double usedSpace = 0.0;
     double maxBaselineDistance = 0.0;
+    double usedSpace = 0.0;
+    if (totalFlex > 0 || alignItems == FlexAlignItems.baseline) {
+      double spacePerFlex = totalFlex > 0 ? (freeSpace / totalFlex) : 0.0;
       child = firstChild;
       while (child != null) {
         int flex = _getFlex(child);
@@ -383,11 +395,45 @@ class RenderFlex extends RenderBox with ContainerRenderObjectMixin<RenderBox, Fl
         assert(child.parentData is FlexBoxParentData);
         child = child.parentData.nextSibling;
       }
+    }
 
-    // Section 8.2: Main Axis Alignment using the justify-content property
+    // Align items along the main axis.
-    double remainingSpace = math.max(0.0, freeSpace - usedSpace);
     double leadingSpace;
     double betweenSpace;
+    double remainingSpace;
+    if (canFlex) {
+      remainingSpace = math.max(0.0, freeSpace - usedSpace);
+      switch (_direction) {
+        case FlexDirection.horizontal:
+          size = constraints.constrain(new Size(mainSize, crossSize));
+          crossSize = size.height;
+          assert(size.width == mainSize);
+          break;
+        case FlexDirection.vertical:
+          size = constraints.constrain(new Size(crossSize, mainSize));
+          crossSize = size.width;
+          assert(size.height == mainSize);
+          break;
+      }
+    } else {
+      leadingSpace = 0.0;
+      betweenSpace = 0.0;
+      switch (_direction) {
+        case FlexDirection.horizontal:
+          size = constraints.constrain(new Size(-_overflow, crossSize));
+          crossSize = size.height;
+          assert(size.width >= -_overflow);
+          remainingSpace = size.width - -_overflow;
+          break;
+        case FlexDirection.vertical:
+          size = constraints.constrain(new Size(crossSize, -_overflow));
+          crossSize = size.width;
+          assert(size.height >= -_overflow);
+          remainingSpace = size.height - -_overflow;
+          break;
+      }
+      _overflow = 0.0;
+    }
     switch (_justifyContent) {
       case FlexJustifyContent.start:
         leadingSpace = 0.0;
@@ -411,17 +457,6 @@ class RenderFlex extends RenderBox with ContainerRenderObjectMixin<RenderBox, Fl
         break;
     }
 
-    switch (_direction) {
-      case FlexDirection.horizontal:
-        size = constraints.constrain(new Size(mainSize, crossSize));
-        crossSize = size.height;
-        break;
-      case FlexDirection.vertical:
-        size = constraints.constrain(new Size(crossSize, mainSize));
-        crossSize = size.width;
-        break;
-    }
-
     // Position elements
     double childMainPosition = leadingSpace;
     child = firstChild;
@@ -517,4 +552,6 @@ class RenderFlex extends RenderBox with ContainerRenderObjectMixin<RenderBox, Fl
     return header;
   }
 
+  String debugDescribeSettings(String prefix) => '${super.debugDescribeSettings(prefix)}${prefix}direction: ${_direction}\n${prefix}justifyContent: ${_justifyContent}\n${prefix}alignItems: ${_alignItems}\n${prefix}textBaseline: ${_textBaseline}\n';
+
 }

packages/flutter/lib/widgets/dialog.dart

@@ -79,7 +79,11 @@ class Dialog extends Component {
     }
 
     if (actions != null)
-      dialogBody.add(new Flex(actions, justifyContent: FlexJustifyContent.end));
+      dialogBody.add(new Container(
+        child: new Flex(actions,
+          justifyContent: FlexJustifyContent.end
+        )
+      ));
 
     return new Stack([
       new Listener(

packages/flutter/lib/widgets/flex.md

@@ -0,0 +1,70 @@
+How To Use Flex In Sky
+======================
+
+Background
+----------
+
+In Sky, widgets are rendered by render boxes. Render boxes are given
+constraints by their parent, and size themselves within those
+constraints. Constraints consist of minimum and maximum widths and
+heights; sizes consist of a specific width and height.
+
+Generally, there are three kinds of boxes, in terms of how they handle
+their constraints:
+
+- Those that try to be as big as possible.
+  For example, the boxes used by `Center` and `Block`.
+- Those that try to be the same size as their children.
+  For example, the boxes used by `Transform` and `Opacity`.
+- Those that try to be a particular size.
+  For example, the boxes used by `Image` and `Text`.
+
+Some widgets, for example `Container`, vary from type to type based on
+their constructor arguments. In the case of `Container`, it defaults
+to trying to be as big as possible, but if you give it a `width`, for
+instance, it tries to honor that and be that particular size.
+
+The constraints are sometimes "tight", meaning that they leave no room
+for the render box to decide on a size (e.g. if the minimum and
+maximum width are the same, it is said to have a tight width). The
+main example of this is the `App` widget, which is contained by the
+`RenderView` class: the box used by the child returned by the
+application's `build` function is given a constraint that forces it to
+exactly fill the application's content area (typically, the entire
+screen).
+
+Unbounded constraints
+---------------------
+
+In certain situations, the constraint that is given to a box will be
+_unbounded_, or infinite. This means that either the maximum width or
+the maximum height is set to `double.INFINITY`.
+
+A box that tries to be as big as possible won't function usefully when
+given an unbounded constraint, and in checked mode, will assert.
+
+The most common cases where a render box finds itself with unbounded
+constraints are within flex boxes (`Row` and `Column`), and **within
+scrollable regions** (mainly `Block`, `ScollableList<T>`, and
+`ScrollableMixedWidgetList`).
+
+Flex
+----
+
+Flex boxes themselves (`Row` and `Column`) behave differently based on
+whether they are in a bounded constraints or unbounded constraints in
+their given direction.
+
+In bounded constraints, they try to be as big as possible in that
+direction.
+
+In unbounded constraints, they try to fit their children in that
+direction. In this case, you cannot set `flex` on the children to
+anything other than 0 (the default). In the widget hierarchy, this
+means that you cannot use `Flexible` when the flex box is inside
+another flex box or inside a scrollable.
+
+In the _cross_ direction, i.e. in their width for `Column` (vertical
+flex) and in their height for `Row` (horizontal flex), they must never
+be unbounded, otherwise they would not be able to reasonably align
+their children.

packages/flutter/lib/widgets/framework.dart

@@ -6,7 +6,6 @@ import 'dart:async';
 import 'dart:collection';
 import 'dart:sky' as sky;
 
-import 'package:sky/base/debug.dart';
 import 'package:sky/base/hit_test.dart';
 import 'package:sky/base/scheduler.dart' as scheduler;
 import 'package:sky/mojo/activity.dart';

packages/flutter/lib/widgets/tool_bar.dart

@@ -46,9 +46,12 @@ class ToolBar extends Component {
     }
 
     List<Widget> children = new List<Widget>();
+
+    // left children
     if (left != null)
       children.add(left);
 
+    // center children (left-aligned, but takes all remaining space)
     children.add(
       new Flexible(
         child: new Padding(
@@ -58,15 +61,21 @@ class ToolBar extends Component {
       )
     );
 
+    // right children
     if (right != null)
       children.addAll(right);
 
     Widget content = new Container(
       child: new DefaultTextStyle(
         style: sideStyle,
-        child: new Flex(
+        child: new Flex([
-          [new Container(child: new Flex(children), height: kToolBarHeight)],
+            new Container(
-          alignItems: FlexAlignItems.end
+              child: new Flex(children),
+              height: kToolBarHeight
+            ),
+          ],
+          direction: FlexDirection.vertical,
+          justifyContent: FlexJustifyContent.end
         )
       ),
       padding: new EdgeDims.symmetric(horizontal: 8.0),