Adjust flex item position to account for reversal when laying out unconstrained

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

@@ -1,4 +1,6 @@
-using Microsoft.Maui.Graphics;
+using System;
+using System.Transactions;
+using Microsoft.Maui.Graphics;
 using Microsoft.Maui.Layouts;
 using NSubstitute;
 using Xunit;
@@ -91,10 +93,14 @@ public void FlexLayoutRecognizesVisibilityChange()
 		 * depending on the target platform.
 		 */
 
-		(IFlexLayout, IView) SetUpUnconstrainedTest()
+		(IFlexLayout, IView) SetUpUnconstrainedTest(Action<FlexLayout> configure = null)
 		{
 			var root = new Grid(); // FlexLayout requires a parent, at least for now
-			var flexLayout = new FlexLayout() as IFlexLayout;
+
+			var controlsFlexLayout = new FlexLayout();
+			configure?.Invoke(controlsFlexLayout);
+
+			var flexLayout = controlsFlexLayout as IFlexLayout;
 
 			var view = Substitute.For<IView>();
 			var size = new Size(100, 100);
@@ -129,5 +135,23 @@ public void UnconstrainedWidthChildrenHaveWidth()
 
 			Assert.Equal(100, flexFrame.Width);
 		}
+
+		[Theory]
+		[InlineData(double.PositiveInfinity, 400, FlexDirection.RowReverse)]
+		[InlineData(double.PositiveInfinity, 400, FlexDirection.Row)]
+		[InlineData(400, double.PositiveInfinity, FlexDirection.ColumnReverse)]
+		[InlineData(400, double.PositiveInfinity, FlexDirection.Column)]
+		public void UnconstrainedMeasureHonorsFlexDirection(double widthConstraint, double heightConstraint,
+			FlexDirection flexDirection)
+		{
+			(var flexLayout, var view) = SetUpUnconstrainedTest((fl) => { fl.Direction = flexDirection; });
+
+			_ = flexLayout.CrossPlatformMeasure(widthConstraint, heightConstraint);
+
+			var flexFrame = flexLayout.GetFlexFrame(view);
+
+			Assert.Equal(0, flexFrame.X);
+			Assert.Equal(0, flexFrame.Y);
+		}
 	}
 }

src/Core/src/Layouts/Flex.cs

@@ -888,6 +888,28 @@ static void layout_items(Item item, int child_begin, int child_end, int children
 
 				layout.lines_sizes += line.size;
 			}
+
+			if (layout.reverse && layout.size_dim == 0)
+			{
+				// Handle reversed layouts when there was no fixed size in the first place. All of the positions will be flipped
+				// across the axis. Luckily the pos variable is already tracking how far negative the values were in this situation,
+				// so we can just offset the distance by that amount and get the desired value
+
+				for (int i = child_begin; i < child_end; i++)
+				{
+					Item child = layout.child_at(item, i);
+					if (!child.IsVisible)
+						continue;
+
+					if (child.Position == Position.Absolute)
+					{
+						// Not helpful for this
+						continue;
+					}
+
+					child.Frame[layout.frame_pos_i] = child.Frame[layout.frame_pos_i] - pos;
+				}
+			}
 		}
 
 		static float absolute_size(float val, float pos1, float pos2, float dim) =>