Bugfix/26633 grid layout manager

src/Core/src/Layouts/GridLayoutManager.cs

@@ -453,6 +453,7 @@ void SecondMeasurePass()
 						{
 							height += _rows[n].Size;
 						}
+						height += _rowSpacing * (cell.RowSpan > 0 ? cell.RowSpan - 1 : 0);
 					}
 
 					if (double.IsInfinity(cell.MeasureWidth))
@@ -465,6 +466,7 @@ void SecondMeasurePass()
 						{
 							width += _columns[n].Size;
 						}
+						width += _columnSpacing * (cell.ColumnSpan > 0 ? cell.ColumnSpan - 1 : 0);
 					}
 
 					if (width == 0 || height == 0)

src/Core/tests/UnitTests/Layouts/GridLayoutManagerTests.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.ComponentModel;
 using System.Diagnostics;
@@ -3235,5 +3235,48 @@ public void StarsAdjustWhenArrangeAndMeasureWidthDiffer(double widthConstraint,
 			AssertArranged(smallerView, new Rect(0, 0, expectedWidth, heightConstraint));
 			AssertArranged(largerView, new Rect(expectedWidth, 0, expectedWidth, heightConstraint));
 		}
-	}
+[Fact]
+public void SpannedCellMeasurementIncludesColumnSpacingIssue26633()
+{
+// Issue: https://github.com/dotnet/maui/issues/26633
+// Grid cells spanning multiple columns with ColumnSpacing > 0 have incorrect measurement
+
+var grid = CreateGridLayout(columns: "auto, *, auto", colSpacing: 6);
+var view = CreateTestView(new Size(50, 50));
+
+SubstituteExtensions.ReturnsForAnyArgs(view.DesiredSize, new Size(50, 50));
+
+SubstituteChildren(grid, view);
+SetLocation(grid, view, row: 0, col: 0, colSpan: 3);
+
+var manager = new GridLayoutManager(grid);
+var measure = manager.Measure(200, double.PositiveInfinity);
+
+// Cell spans all 3 columns: should get full grid width of 200px
+// Without fix: gets 188px (missing spacing in calculation)
+view.Received().Measure(Arg.Is<double>(width => width == 200), Arg.Any<double>());
+}
+
+[Fact]
+public void SpannedCellMeasurementIncludesRowSpacingIssue26633()
+{
+// Issue: https://github.com/dotnet/maui/issues/26633  
+// Grid cells spanning multiple rows with RowSpacing > 0 have incorrect measurement
+
+var grid = CreateGridLayout(rows: "auto, *, auto", rowSpacing: 6);
+var view = CreateTestView(new Size(50, 50));
+
+SubstituteExtensions.ReturnsForAnyArgs(view.DesiredSize, new Size(50, 50));
+
+SubstituteChildren(grid, view);
+SetLocation(grid, view, row: 0, col: 0, rowSpan: 3);
+
+var manager = new GridLayoutManager(grid);
+var measure = manager.Measure(double.PositiveInfinity, 200);
+
+// Cell spans all 3 rows: should get full grid width of 200px
+// Without fix: gets 188px (missing spacing in calculation)
+view.Received().Measure(Arg.Any<double>(), Arg.Is<double>(height => height == 200));
+}
 }
+}