Allow children to overflow above a Baseline widget (#3573)

Turns out a common use for Baseline is making sure that the child's
baseline is higher than it otherwise would be, e.g. with the Material
font or other symbols.

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

@@ -824,9 +824,19 @@ class RenderCustomSingleChildLayoutBox extends RenderShiftedBox {
 /// Shifts the child down such that the child's baseline (or the
 /// bottom of the child, if the child has no baseline) is [baseline]
 /// logical pixels below the top of this box, then sizes this box to
-/// contain the child. If [baseline] is less than the distance from
-/// the top of the child to the baseline of the child, then the child
-/// is top-aligned instead.
+/// contain the child.
+///
+/// If [baseline] is less than the distance from the top of the child
+/// to the baseline of the child, then the child will overflow the top
+/// of the box. This is typically not desireable, in particular, that
+/// part of the child will not be found when doing hit tests, so the
+/// user cannot interact with that part of the child.
+///
+/// This box will be sized so that its bottom is coincident with the
+/// bottom of the child. This means if this box shifts the child down,
+/// there will be space between the top of this box and the top of the
+/// child, but there is never space between the bottom of the child
+/// and the bottom of the box.
 class RenderBaseline extends RenderShiftedBox {
   /// Creates a [RenderBaseline] object.
   ///
@@ -870,7 +880,7 @@ class RenderBaseline extends RenderShiftedBox {
     if (child != null) {
       child.layout(constraints.loosen(), parentUsesSize: true);
       final double childBaseline = child.getDistanceToBaseline(baselineType);
-      final double actualBaseline = math.max(baseline, childBaseline);
+      final double actualBaseline = baseline;
       final double top = actualBaseline - childBaseline;
       final BoxParentData childParentData = child.parentData;
       childParentData.offset = new Offset(0.0, top);

packages/flutter/test/rendering/baseline_test.dart

+// 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 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
+import 'package:test/test.dart';
+
+import 'rendering_tester.dart';
+
+void main() {
+  test("RenderBaseline", () {
+    RenderBaseline parent;
+    RenderSizedBox child;
+    RenderBox root = new RenderPositionedBox(
+      alignment: FractionalOffset.topLeft,
+      child: parent = new RenderBaseline(
+        baseline: 0.0,
+        baselineType: TextBaseline.alphabetic,
+        child: child = new RenderSizedBox(new Size(100.0, 100.0))
+      )
+    );
+    BoxParentData childParentData = child.parentData;
+
+    layout(root, phase: EnginePhase.layout);
+    expect(childParentData.offset.dx, equals(0.0));
+    expect(childParentData.offset.dy, equals(-100.0));
+    expect(parent.size, equals(new Size(100.0, 0.0)));
+
+    parent.baseline = 25.0;
+    layout(root, phase: EnginePhase.layout);
+    expect(childParentData.offset.dx, equals(0.0));
+    expect(childParentData.offset.dy, equals(-75.0));
+    expect(parent.size, equals(new Size(100.0, 25.0)));
+
+    parent.baseline = 90.0;
+    layout(root, phase: EnginePhase.layout);
+    expect(childParentData.offset.dx, equals(0.0));
+    expect(childParentData.offset.dy, equals(-10.0));
+    expect(parent.size, equals(new Size(100.0, 90.0)));
+
+    parent.baseline = 100.0;
+    layout(root, phase: EnginePhase.layout);
+    expect(childParentData.offset.dx, equals(0.0));
+    expect(childParentData.offset.dy, equals(0.0));
+    expect(parent.size, equals(new Size(100.0, 100.0)));
+
+    parent.baseline = 110.0;
+    layout(root, phase: EnginePhase.layout);
+    expect(childParentData.offset.dx, equals(0.0));
+    expect(childParentData.offset.dy, equals(10.0));
+    expect(parent.size, equals(new Size(100.0, 110.0)));
+  });
+}

packages/flutter/test/rendering/rendering_tester.dart

@@ -105,3 +105,15 @@ class TestCallbackPainter extends CustomPainter {
   @override
   bool shouldRepaint(TestCallbackPainter oldPainter) => true;
 }
+
+
+class RenderSizedBox extends RenderBox {
+  RenderSizedBox(this._size);
+
+  final Size _size;
+
+  @override
+  void performLayout() {
+    size = constraints.constrain(_size);
+  }
+}