Ensure that AbsoluteLayout measures children at their target sizes (#3322)

src/Core/src/Layouts/AbsoluteLayoutManager.cs

@@ -33,14 +33,16 @@ namespace Microsoft.Maui.Layouts
 					continue;
 				}
 
-				var measure = child.Measure(availableWidth, availableHeight);
-
 				var bounds = AbsoluteLayout.GetLayoutBounds(child);
 				var flags = AbsoluteLayout.GetLayoutFlags(child);
-
 				bool isWidthProportional = HasFlag(flags, AbsoluteLayoutFlags.WidthProportional);
 				bool isHeightProportional = HasFlag(flags, AbsoluteLayoutFlags.HeightProportional);
 
+				var measureWidth = ResolveChildMeasureConstraint(bounds.Width, isWidthProportional, widthConstraint);
+				var measureHeight = ResolveChildMeasureConstraint(bounds.Height, isHeightProportional, heightConstraint);
+
+				var measure = child.Measure(measureWidth, measureHeight);
+
 				var width = ResolveDimension(isWidthProportional, bounds.Width, availableWidth, measure.Width);
 				var height = ResolveDimension(isHeightProportional, bounds.Height, availableHeight, measure.Height);
 
@@ -122,5 +124,21 @@ namespace Microsoft.Maui.Layouts
 
 			return value;
 		}
+
+		static double ResolveChildMeasureConstraint(double boundsValue, bool proportional, double constraint) 
+		{
+			if (boundsValue < 0)
+			{
+				// If the child view doesn't have bounds set by the AbsoluteLayout, then we'll measure using the full constraint value
+				return constraint;
+			}
+
+			if (proportional)
+			{
+				return boundsValue * constraint;
+			}
+
+			return boundsValue;
+		}
 	}
 }

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

@@ -405,5 +405,45 @@ namespace Microsoft.Maui.UnitTests.Layouts
 			Assert.Equal(arrangedWidth, actual.Width);
 			Assert.Equal(arrangedHeight, actual.Height);
 		}
+
+		[Fact]
+		public void ChildMeasureRespectsAbsoluteBounds() 
+		{
+			double expectedWidth = 115;
+			double expectedHeight = 230;
+
+			var abs = CreateTestLayout();
+			var child = CreateTestView();
+			SubstituteChildren(abs, child);
+			var childBounds = new Rectangle(0, 0, expectedWidth, expectedHeight);
+			SetLayoutBounds(abs, child, childBounds);
+
+			var gridLayoutManager = new AbsoluteLayoutManager(abs);
+			var measure = gridLayoutManager.Measure(double.PositiveInfinity, double.PositiveInfinity);
+
+			child.Received().Measure(Arg.Is(expectedWidth), Arg.Is(expectedHeight));
+		}
+
+		[Fact]
+		public void ChildMeasureRespectsProportionalBounds()
+		{
+			double expectedWidth = 0.5;
+			double expectedHeight = 0.6;
+
+			double widthConstraint = 200;
+			double heightConstraint = 200;
+
+			var abs = CreateTestLayout();
+			var child = CreateTestView();
+			SubstituteChildren(abs, child);
+			var childBounds = new Rectangle(0, 0, expectedWidth, expectedHeight);
+			SetLayoutBounds(abs, child, childBounds);
+			SetLayoutFlags(abs, child, AbsoluteLayoutFlags.SizeProportional);
+
+			var gridLayoutManager = new AbsoluteLayoutManager(abs);
+			var measure = gridLayoutManager.Measure(widthConstraint, heightConstraint);
+
+			child.Received().Measure(Arg.Is(expectedWidth * widthConstraint), Arg.Is(expectedHeight * heightConstraint));
+		}
 	}
 }