When the available length is not enough, clip the measure list.

src/Avalonia.Controls/Grid.cs

@@ -211,10 +211,8 @@ namespace Avalonia.Controls
             {
                 var (column, columnSpan) = safeColumns[child];
                 var (row, rowSpan) = safeRows[child];
-                var width = Enumerable.Range(column, columnSpan)
+                var width = Enumerable.Range(column, columnSpan).Select(x => columnResult.LengthList[x]).Sum();
-                    .Select(x => columnResult.LengthList[x].Length.Value).Sum();
+                var height = Enumerable.Range(row, rowSpan).Select(x => rowResult.LengthList[x]).Sum();
-                var height = Enumerable.Range(row, rowSpan)
-                    .Select(x => rowResult.LengthList[x].Length.Value).Sum();
 
                 MeasureOnce(child, new Size(width, height));
             }
@@ -256,10 +254,8 @@ namespace Avalonia.Controls
             {
                 var (column, columnSpan) = safeColumns[child];
                 var (row, rowSpan) = safeRows[child];
-                var width = Enumerable.Range(column, columnSpan)
+                var width = Enumerable.Range(column, columnSpan).Select(x => columnResult.LengthList[x]).Sum();
-                    .Select(x => columnResult.LengthList[x].Length.Value).Sum();
+                var height = Enumerable.Range(row, rowSpan).Select(x => rowResult.LengthList[x]).Sum();
-                var height = Enumerable.Range(row, rowSpan)
-                    .Select(x => rowResult.LengthList[x].Length.Value).Sum();
 
                 child.Arrange(new Rect(0, 0, width, height));
             }

src/Avalonia.Controls/Utils/GridLayout.cs

@@ -176,9 +176,11 @@ namespace Avalonia.Controls.Utils
 
             // M6/6. Expand all the left stars. These stars have no conventions or only have max value so they can be expanded from zero to constrant.
             var dynamicConvention = ExpandStars(conventions, containerLength);
+            Clip(dynamicConvention, containerLength);
 
-            // Stores the measure result.
+            // Stores the measuring result.
-            return new MeasureResult(containerLength, desiredLength, greedyDesiredLength, conventions, dynamicConvention);
+            return new MeasureResult(containerLength, desiredLength, greedyDesiredLength,
+                conventions, dynamicConvention);
         }
 
         public ArrangeResult Arrange(double finalLength, MeasureResult measure)
@@ -201,7 +203,7 @@ namespace Avalonia.Controls.Utils
         }
 
         [Pure]
-        private static List<LengthConvention> ExpandStars(IEnumerable<LengthConvention> conventions, double constraint)
+        private static List<double> ExpandStars(IEnumerable<LengthConvention> conventions, double constraint)
         {
             // Initial.
             var dynamicConvention = conventions.Select(x => x.Clone()).ToList();
@@ -244,7 +246,25 @@ namespace Avalonia.Controls.Utils
 
             Debug.Assert(dynamicConvention.All(x => x.Length.IsAbsolute));
 
-            return dynamicConvention;
+            return dynamicConvention.Select(x => x.Length.Value).ToList();
+        }
+
+        private static void Clip(IList<double> lengthList, double constraint)
+        {
+            var measureLength = 0.0;
+            for (var i = 0; i < lengthList.Count; i++)
+            {
+                var length = lengthList[i];
+                if (constraint - measureLength > length)
+                {
+                    measureLength += length;
+                }
+                else
+                {
+                    lengthList[i] = constraint - measureLength;
+                    measureLength = constraint;
+                }
+            }
         }
 
         internal class LengthConvention : ICloneable
@@ -299,13 +319,13 @@ namespace Avalonia.Controls.Utils
         internal class MeasureResult
         {
             internal MeasureResult(double containerLength, double desiredLength, double greedyDesiredLength,
-                IReadOnlyList<LengthConvention> leanConventions, IReadOnlyList<LengthConvention> expandedConventions)
+                IReadOnlyList<LengthConvention> leanConventions, IReadOnlyList<double> expandedConventions)
             {
                 ContainerLength = containerLength;
                 DesiredLength = desiredLength;
                 GreedyDesiredLength = greedyDesiredLength;
                 LeanLengthList = leanConventions;
-                LengthList = expandedConventions.Select(x => x.Length.Value).ToList();
+                LengthList = expandedConventions;
             }
 
             public double ContainerLength { get; }

tests/Avalonia.Controls.UnitTests/GridLayoutTests.cs

@@ -9,6 +9,8 @@ namespace Avalonia.Controls.UnitTests
     {
         [Theory]
         [InlineData("100, 200, 300", 800d, 600d, new[] { 100d, 200d, 300d })]
+        [InlineData("100, 200, 300", 600d, 600d, new[] { 100d, 200d, 300d })]
+        [InlineData("100, 200, 300", 400d, 400d, new[] { 100d, 200d, 100d })]
         public void MeasureArrange_AllPixelLength_Correct(string length, double containerLength,
             double expectedDesiredLength, IList<double> expectedLengthList)
         {
@@ -55,19 +57,6 @@ namespace Avalonia.Controls.UnitTests
             TestRowDefinitionsOnly(length, containerLength, expectedDesiredLength, expectedLengthList);
         }
 
-        [Fact]
-        public void MeasureArrange_AllPixelLengthButNotEnough_Correct()
-        {
-            // Arrange
-            var layout = new GridLayout(new RowDefinitions("100,200,300"));
-
-            // Measure - Action & Assert
-            var measure = layout.Measure(400);
-            Assert.Equal(new[] { 100d, 200d, 300d }, measure.LengthList);
-
-            // Arrange - Action & Assert
-        }
-
         [SuppressMessage("ReSharper", "ParameterOnlyUsedForPreconditionCheck.Local")]
         private static void TestRowDefinitionsOnly(string length, double containerLength,
             double expectedDesiredLength, IList<double> expectedLengthList)