Fix Grid sizing being broken when using Grid.ColumnSpan.

Caused by CreateMatrices not clearing the existing matrices. This means previous measures alter the results of a measure.

src/Perspex.Controls/Grid.cs

@@ -587,6 +587,11 @@ namespace Perspex.Controls
                 _rowMatrix = new Segment[rowCount, rowCount];
                 _colMatrix = new Segment[colCount, colCount];
             }
+            else
+            {
+                Array.Clear(_rowMatrix, 0, _rowMatrix.Length);
+                Array.Clear(_colMatrix, 0, _colMatrix.Length);
+            }
         }
 
         private void ExpandStarCols(Size availableSize)

tests/Perspex.Controls.UnitTests/GridTests.cs

@@ -0,0 +1,68 @@
+// 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.Controls.UnitTests
+{
+    public class GridTests
+    {
+        [Fact]
+        public void Calculates_Colspan_Correctly()
+        {
+            var target = new Grid
+            {
+                ColumnDefinitions = new ColumnDefinitions
+                {
+                    new ColumnDefinition(GridLength.Auto),
+                    new ColumnDefinition(new GridLength(4, GridUnitType.Pixel)),
+                    new ColumnDefinition(GridLength.Auto),
+                },
+                RowDefinitions = new RowDefinitions
+                {
+                    new RowDefinition(GridLength.Auto),
+                    new RowDefinition(GridLength.Auto),
+                },
+                Children = new Controls
+                {
+                    new Border
+                    {
+                        Width = 100,
+                        Height = 25,
+                        [Grid.ColumnSpanProperty] = 3,
+                    },
+                    new Border
+                    {
+                        Width = 150,
+                        Height = 25,
+                        [Grid.RowProperty] = 1,
+                    },
+                    new Border
+                    {
+                        Width = 50,
+                        Height = 25,
+                        [Grid.RowProperty] = 1,
+                        [Grid.ColumnProperty] = 2,
+                    }
+                },
+            };
+
+            target.Measure(Size.Infinity);
+
+            // Issue #25 only appears after a second measure
+            target.InvalidateMeasure();
+            target.Measure(Size.Infinity);
+
+            target.Arrange(new Rect(target.DesiredSize));
+
+            Assert.Equal(new Size(204, 50), target.Bounds.Size);
+            Assert.Equal(150d, target.ColumnDefinitions[0].ActualWidth);
+            Assert.Equal(4d, target.ColumnDefinitions[1].ActualWidth);
+            Assert.Equal(50d, target.ColumnDefinitions[2].ActualWidth);
+            Assert.Equal(new Rect(52, 0, 100, 25), target.Children[0].Bounds);
+            Assert.Equal(new Rect(0, 25, 150, 25), target.Children[1].Bounds);
+            Assert.Equal(new Rect(154, 25, 50, 25), target.Children[2].Bounds);
+        }
+    }
+}

tests/Perspex.Controls.UnitTests/Perspex.Controls.UnitTests.csproj

@@ -84,6 +84,7 @@
     <Compile Include="ClassesTests.cs" />
     <Compile Include="EnumerableExtensions.cs" />
     <Compile Include="GridSplitterTests.cs" />
+    <Compile Include="GridTests.cs" />
     <Compile Include="HeaderedItemsControlTests .cs" />
     <Compile Include="ControlTests_NameScope.cs" />
     <Compile Include="Generators\ItemContainerGeneratorTests.cs" />