Snapshot grid structure info instead of modifying it in place

Fixes #6247; Fixes #10470

Xamarin.Forms.Core/Grid.cs

@@ -162,14 +162,14 @@ internal override void ComputeConstraintForView(View view)
 
 			var result = LayoutConstraint.None;
 
-			if (_rows == null || _columns == null)
-				EnsureRowsColumnsInitialized();
+			// grab a snapshot of this grid's structure for computing the constraints
+			var structure = new GridStructure(this);
 
 			if (vOptions.Alignment == LayoutAlignment.Fill)
 			{
 				int row = GetRow(view);
 				int rowSpan = GetRowSpan(view);
-				List<RowDefinition> rowDefinitions = _rows;
+				List<RowDefinition> rowDefinitions = structure.Rows;
 
 				var canFix = true;
 
@@ -196,7 +196,7 @@ internal override void ComputeConstraintForView(View view)
 			{
 				int col = GetColumn(view);
 				int colSpan = GetColumnSpan(view);
-				List<ColumnDefinition> columnDefinitions = _columns;
+				List<ColumnDefinition> columnDefinitions = structure.Columns;
 
 				var canFix = true;
 
@@ -227,13 +227,7 @@ public void InvalidateMeasureInernalNonVirtual(InvalidationTrigger trigger)
 		{
 			InvalidateMeasureInternal(trigger);
 		}
-		internal override void InvalidateMeasureInternal(InvalidationTrigger trigger)
-		{
-			base.InvalidateMeasureInternal(trigger);
-			_columns = null;
-			_rows = null;
-		}
-
+
 		void OnDefinitionChanged(object sender, EventArgs args)
 		{
 			ComputeConstrainsForChildren();