Constrain to availableSize in MeasureCore.

#2431 erroneously removed the `.Constrain(availableSize)` call in `Layoutable.Measure`. Now that the WPF source is available, I can see i that WPF does in fact constrain measure to availableSize and Grid relies on this.

Put constraint back in, undo the changes to the controls changed in #2431 (`StackPanel`, `Image`) and update the expected test results based on cross-checks with WPF in https://github.com/wieslawsoltes/WpfUnitTests/pull/1.

src/Avalonia.Controls/Image.cs

@@ -96,7 +96,7 @@ namespace Avalonia.Controls
                 }
             }
 
-            return result.Constrain(availableSize);
+            return result;
         }
 
         /// <inheritdoc/>

src/Avalonia.Controls/StackPanel.cs

@@ -229,9 +229,7 @@ namespace Avalonia.Controls
                 stackDesiredSize = stackDesiredSize.WithHeight(stackDesiredSize.Height - (hasVisibleChild ? spacing : 0));
             }
 
-            // TODO: In WPF `.Constrain(availableSize)` is not used.
+            return stackDesiredSize;
-            //return stackDesiredSize;
-            return stackDesiredSize.Constrain(availableSize); 
         }
 
         /// <summary>

src/Avalonia.Layout/Layoutable.cs

@@ -534,6 +534,9 @@ namespace Avalonia.Layout
                 height = Math.Min(height, MaxHeight);
                 height = Math.Max(height, MinHeight);
 
+                width = Math.Min(width, availableSize.Width);
+                height = Math.Min(height, availableSize.Height);
+
                 if (UseLayoutRounding)
                 {
                     var scale = GetLayoutScale();

tests/Avalonia.Controls.UnitTests/StackPanelTests.cs

@@ -210,13 +210,13 @@ namespace Avalonia.Controls.UnitTests
                 new[]
                 {
                     new Rect(0, 0, 50, 10),
-                    new Rect(0, 10, 150, 10),
+                    new Rect(0, 10, 100, 10),
                     new Rect(25, 20, 50, 10),
-                    new Rect(-25, 30, 150, 10),
+                    new Rect(0, 30, 100, 10),
                     new Rect(50, 40, 50, 10),
-                    new Rect(-50, 50, 150, 10),
+                    new Rect(0, 50, 100, 10),
                     new Rect(0, 60, 100, 10),
-                    new Rect(0, 70, 150, 10),
+                    new Rect(0, 70, 100, 10),
 
                 }, bounds);
         }
@@ -283,13 +283,13 @@ namespace Avalonia.Controls.UnitTests
                 new[]
                 {
                     new Rect(0, 0, 10, 50),
-                    new Rect(10, 0, 10, 150),
+                    new Rect(10, 0, 10, 100),
                     new Rect(20, 25, 10, 50),
-                    new Rect(30, -25, 10, 150),
+                    new Rect(30, 0, 10, 100),
                     new Rect(40, 50, 10, 50),
-                    new Rect(50, -50, 10, 150),
+                    new Rect(50, 0, 10, 100),
                     new Rect(60, 0, 10, 100),
-                    new Rect(70, 0, 10, 150),
+                    new Rect(70, 0, 10, 100),
                 }, bounds);
         }