Make Arrange call Measure if !IsMeasureValid.

To follow WPF. Also removed force parameter from Arrange and fixed
Decorator measure invalidation.

src/Perspex.Controls/Decorator.cs

@@ -27,6 +27,7 @@ namespace Perspex.Controls
         /// </summary>
         static Decorator()
         {
+            AffectsMeasure(ChildProperty, PaddingProperty);
             ChildProperty.Changed.AddClassHandler<Decorator>(x => x.ChildChanged);
         }
 

src/Perspex.Layout/ILayoutable.cs

@@ -93,11 +93,7 @@ namespace Perspex.Layout
         /// Arranges the control and its children.
         /// </summary>
         /// <param name="rect">The control's new bounds.</param>
-        /// <param name="force">
-        /// If true, the control will be arranged even if <paramref name="rect"/> has not changed
-        /// from the last arrange.
-        /// </param>
-        void Arrange(Rect rect, bool force = false);
+        void Arrange(Rect rect);
 
         /// <summary>
         /// Invalidates the measurement of the control and queues a new layout pass.

src/Perspex.Layout/Layoutable.cs

@@ -352,11 +352,7 @@ namespace Perspex.Layout
         /// Arranges the control and its children.
         /// </summary>
         /// <param name="rect">The control's new bounds.</param>
-        /// <param name="force">
-        /// If true, the control will be arranged even if <paramref name="rect"/> has not changed
-        /// from the last arrange.
-        /// </param>
-        public void Arrange(Rect rect, bool force = false)
+        public void Arrange(Rect rect)
         {
             if (IsInvalidRect(rect))
             {
@@ -367,10 +363,10 @@ namespace Perspex.Layout
             // be re-run.
             if (!IsMeasureValid)
             {
-                return;
+                Measure(rect.Size);
             }
 
-            if (force || !IsArrangeValid || _previousArrange != rect)
+            if (!IsArrangeValid || _previousArrange != rect)
             {
                 _layoutLog.Verbose("Arrange to {Rect} ", rect);
 

tests/Perspex.Layout.UnitTests/ArrangeTests.cs

@@ -0,0 +1,33 @@
+// Copyright (c) The Perspex Project. All rights reserved.
+// Licensed under the MIT license. See licence.md file in the project root for full license information.
+
+using Perspex.Controls;
+using Xunit;
+
+namespace Perspex.Layout.UnitTests
+{
+    public class ArrangeTests
+    {
+        [Fact]
+        public void Arrange_With_IsMeasureValid_False_Calls_Measure()
+        {
+            var target = new TestControl();
+
+            Assert.False(target.IsMeasureValid);
+            target.Arrange(new Rect(0, 0, 120, 120));
+            Assert.True(target.IsMeasureValid);
+            Assert.Equal(new Size(120, 120), target.MeasureConstraint);
+        }
+
+        private class TestControl : Border
+        {
+            public Size MeasureConstraint { get; private set; }
+
+            protected override Size MeasureOverride(Size constraint)
+            {
+                MeasureConstraint = constraint;
+                return base.MeasureOverride(constraint);
+            }
+        }
+    }
+}

tests/Perspex.Layout.UnitTests/Perspex.Layout.UnitTests.csproj

@@ -87,6 +87,7 @@
   <ItemGroup>
     <Compile Include="FullLayoutTests.cs" />
     <Compile Include="LayoutManagerTests.cs" />
+    <Compile Include="ArrangeTests.cs" />
     <Compile Include="MeasureTests.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="TestLayoutRoot.cs" />

tests/Perspex.SceneGraph.UnitTests/VisualTree/BoundsTrackerTests.cs

@@ -44,9 +44,9 @@ namespace Perspex.SceneGraph.UnitTests.VisualTree
 
             tree.Padding = new Thickness(15);
             tree.Measure(Size.Infinity);
-            tree.Arrange(new Rect(0, 0, 100, 100), true);
+            tree.Arrange(new Rect(0, 0, 100, 100));
 
-            Assert.Equal(new Rect(47, 47, 15, 15), results[1].Bounds);
+            Assert.Equal(new Rect(37, 37, 15, 15), results[1].Bounds);
         }
     }
 }