Fix grid desired size error (#18507)

* fix desired size error

* cache RowSpacing and ColumnSpacing

* add unittest

---------

Co-authored-by: Poker <poker_sang@outlook.com>

src/Avalonia.Controls/Grid.cs

@@ -446,6 +446,8 @@ namespace Avalonia.Controls
                     //
 
                     MeasureCellsGroup(extData.CellGroup1, constraint, false, false);
+                    double rowSpacing = RowSpacing;
+                    double columnSpacing = ColumnSpacing;
                     double combinedRowSpacing = RowSpacing * (RowDefinitions.Count - 1);
                     double combinedColumnSpacing = ColumnSpacing * (ColumnDefinitions.Count - 1);
                     Size innerAvailableSize = new Size(constraint.Width - combinedColumnSpacing, constraint.Height - combinedRowSpacing);
@@ -513,7 +515,7 @@ namespace Avalonia.Controls
 
                     gridDesiredSize = new Size(
                         CalculateDesiredSize(DefinitionsU) + ColumnSpacing * (DefinitionsU.Count - 1),
-                        CalculateDesiredSize(DefinitionsV) + RowSpacing * (DefinitionsU.Count - 1));
+                        CalculateDesiredSize(DefinitionsV) + RowSpacing * (DefinitionsV.Count - 1));
                 }
             }
             finally
@@ -547,8 +549,8 @@ namespace Avalonia.Controls
                 else
                 {
                     Debug.Assert(DefinitionsU.Count > 0 && DefinitionsV.Count > 0);
-                    double columnSpacing = ColumnSpacing;
                     double rowSpacing = RowSpacing;
+                    double columnSpacing = ColumnSpacing;
                     double combinedRowSpacing = rowSpacing * (RowDefinitions.Count - 1);
                     double combinedColumnSpacing = columnSpacing * (ColumnDefinitions.Count - 1);
                     SetFinalSize(DefinitionsU, arrangeSize.Width - combinedColumnSpacing, true);

tests/Avalonia.Controls.UnitTests/GridTests.cs

@@ -1768,12 +1768,16 @@ namespace Avalonia.Controls.UnitTests
         {
             var target = new Grid
             {
-                Height = 100,
                 ColumnSpacing = 20,
+                RowDefinitions = RowDefinitions.Parse("Auto"),
                 ColumnDefinitions = ColumnDefinitions.Parse("20,20"),
                 Children =
                 {
-                    new Border { [Grid.ColumnSpanProperty] = 2 }
+                    new Border 
+                    {
+                        Height = 100,
+                        [Grid.ColumnSpanProperty] = 2
+                    }
                 },
             };
             target.Measure(new Size(100, 100));