When measuring FlexLayout unconstrained, allow child to be desired si…

…ze (#13216)

* When measuring FlexLayout unconstrained, allow child to be desired size (rather than zero)
Fixes #11909

* Fix issue on Windows with HorizontalStackLayout

* De-duplicate some test code

src/Controls/src/Core/Layout/FlexLayout.cs

@@ -483,6 +483,14 @@ void AddFlexItem(IView child)
 		{
 			if (_root == null)
 				return;
+
+			if (child is not BindableObject)
+			{
+				// If this is a pure Core IView, we need to track all the flex properties
+				// locally because we don't have attached properties for them
+				_viewInfo.Add(child, new FlexInfo());
+			}
+
 			var item = (child as FlexLayout)?._root ?? new Flex.Item();
 			InitItemProperties(child, item);
 			if (child is not FlexLayout)

src/Controls/tests/Core.UnitTests/Layouts/FlexLayoutTests.cs

@@ -1,6 +1,7 @@
 using Microsoft.Maui.Graphics;
 using Microsoft.Maui.Layouts;
 using Xunit;
+using NSubstitute;
 
 namespace Microsoft.Maui.Controls.Core.UnitTests.Layouts
 {
@@ -80,5 +81,53 @@ public void FlexLayoutRecognizesVisibilityChange()
 			// now that the first view is not visible
 			Assert.True(whenInvisible != whenVisible);
 		}
+
+		/*
+		 * These next two tests deal with unconstrained measure of FlexLayout. Be default, FL
+		 * wants to stretch children across each axis. But you can't stretch things across infinity
+		 * without it getting weird. So for _measurement_ purposes, we treat infinity as zero and 
+		 * just give the children their desired size in the unconstrained direction. Otherwise, FL
+		 * would just set their flex frame sizes to zero, which can either cause blanks or layout cycles,
+		 * depending on the target platform.
+		 */
+
+		(IFlexLayout, IView) SetUpUnconstrainedTest() 
+		{
+			var root = new Grid(); // FlexLayout requires a parent, at least for now
+			var flexLayout = new FlexLayout() as IFlexLayout;
+
+			var view = Substitute.For<IView>();
+			var size = new Size(100, 100);
+			view.Measure(Arg.Any<double>(), Arg.Any<double>()).Returns(size);
+
+			root.Add(flexLayout);
+			flexLayout.Add(view);
+
+			return (flexLayout, view);
+		}
+
+		[Fact]
+		public void UnconstrainedHeightChildrenHaveHeight()
+		{
+			(var flexLayout, var view) = SetUpUnconstrainedTest();
+
+			_ = flexLayout.CrossPlatformMeasure(400, double.PositiveInfinity);
+
+			var flexFrame = flexLayout.GetFlexFrame(view);
+
+			Assert.Equal(100, flexFrame.Height);
+		}
+
+		[Fact]
+		public void UnconstrainedWidthChildrenHaveWidth()
+		{
+			(var flexLayout, var view) = SetUpUnconstrainedTest();
+
+			_ = flexLayout.CrossPlatformMeasure(double.PositiveInfinity, 400);
+
+			var flexFrame = flexLayout.GetFlexFrame(view);
+
+			Assert.Equal(100, flexFrame.Width);
+		}
 	}
 }

src/Controls/tests/DeviceTests/Elements/Layout/LayoutTests.cs

@@ -3,7 +3,9 @@
 using Microsoft.Maui.Controls;
 using Microsoft.Maui.Graphics;
 using Microsoft.Maui.Handlers;
+using Microsoft.Maui.Platform;
 using Xunit;
+using Xunit.Sdk;
 
 namespace Microsoft.Maui.DeviceTests
 {
@@ -151,5 +153,38 @@ static void CreateLayout(Type layoutType, out Layout layout, out Label label)
 				layout.Add(label);
 			}
 		}
+
+		[Fact, Category(TestCategory.FlexLayout)]
+		public async Task FlexLayoutInVerticalStackLayoutDoesNotCycle() 
+		{
+			await FlexLayoutInStackLayoutDoesNotCycle(new VerticalStackLayout());
+		}
+
+		[Fact, Category(TestCategory.FlexLayout)]
+		public async Task FlexLayoutInHorizontalStackLayoutDoesNotCycle()
+		{
+			await FlexLayoutInStackLayoutDoesNotCycle(new HorizontalStackLayout());
+		}
+
+		async Task FlexLayoutInStackLayoutDoesNotCycle(IStackLayout root)
+		{
+			var flexLayout = new FlexLayout();
+			var label = new Label { Text = "Hello" };
+
+			flexLayout.Add(label);
+			root.Add(flexLayout);
+
+			await InvokeOnMainThreadAsync(async () =>
+			{
+				var labelHandler = CreateHandler<LabelHandler>(label);
+				var flexLayoutHandler = CreateHandler<LayoutHandler>(flexLayout);
+				var layoutHandler = CreateHandler<LayoutHandler>(root);
+
+				// If this can be attached to the hierarchy and make it through a layout 
+				// without crashing, then we're good.
+
+				await root.ToPlatform(MauiContext).AttachAndRun(() => { });
+			});
+		}
 	}
 }

src/Controls/tests/DeviceTests/TestCategory.cs

@@ -17,8 +17,9 @@ public static class TestCategory
 		public const string Editor = "Editor";
 		public const string Element = "Element";
 		public const string Entry = "Entry";
-		public const string Frame = "Frame";
+		public const string FlexLayout = "FlexLayout";
 		public const string FlyoutPage = "FlyoutPage";
+		public const string Frame = "Frame";
 		public const string Gesture = "Gesture";
 		public const string Image = "Image";
 		public const string Label = "Label";

src/Core/src/Layouts/Flex.cs

@@ -530,7 +530,7 @@ static void layout_item(Item item, float width, float height)
 					for (int j = 0; j < 2; j++)
 					{
 						int size_off = j + 2;
-						if (size_off == layout.frame_size2_i && child_align(child, item) == AlignItems.Stretch)
+						if (size_off == layout.frame_size2_i && child_align(child, item) == AlignItems.Stretch && layout.align_dim > 0)
 							continue;
 						float val = size[j];
 						if (!float.IsNaN(val))
@@ -579,7 +579,13 @@ static void layout_item(Item item, float width, float height)
 				layout.flex_grows += child.Grow;
 				layout.flex_shrinks += child.Shrink;
 
-				layout.flex_dim -= child_size + child.MarginThickness(layout.vertical);
+				if(layout.flex_dim > 0) 
+				{
+					// If flex_dim is zero, it's because we're measuring unconstrained in that direction
+					// So we don't need to keep a running tally of available space
+
+					layout.flex_dim -= child_size + child.MarginThickness(layout.vertical);
+				}
 
 				relative_children_count++;