[patch] Stretch StackPanel children across the cross axis so alignment applies

TUI.Test/StackPanelTests.cs

@@ -510,11 +510,64 @@ public void StackPanelVerticalMeasuresWidthIndependentElementsAtTheirOwnSize()
 		FixedSizeElementBase derived = new() { Dimensions = new Dimensions(4, 3) };
 
 		Assert.AreEqual(direct.Size, ((IUIElement)direct).CalculateRequiredDimensions(1));
-		Assert.AreEqual(new Dimensions(3, 2), direct.Dimensions);
+		// The height is the measured one; the width spans the panel (ktsu-dev/TUI#158)
+		Assert.AreEqual(new Dimensions(10, 2), direct.Dimensions);
 		Assert.AreEqual(new Position(0, 2), next.Position);
 		Assert.AreEqual(derived.CalculateRequiredDimensions(), derived.CalculateRequiredDimensions(1));
 	}
 
+	/// <summary>
+	/// A vertical stack gives each child the panel's full width, so a centered or right-aligned
+	/// child has room to align in (ktsu-dev/TUI#158).
+	/// </summary>
+	/// <param name="alignment">The horizontal alignment under test.</param>
+	/// <param name="expectedX">The column the text should be written at.</param>
+	[TestMethod]
+	[DataRow(HorizontalAlignment.Left, 0)]
+	[DataRow(HorizontalAlignment.Center, 19)]
+	[DataRow(HorizontalAlignment.Right, 38)]
+	public void StackPanelVerticalChildAlignsWithinThePanelWidth(HorizontalAlignment alignment, int expectedX)
+	{
+		// Arrange
+		TextElement text = new("hi") { HorizontalAlignment = alignment };
+		StackPanel panel = [text];
+		panel.Position = Position.Origin;
+		panel.Dimensions = new Dimensions(40, 10);
+		RecordingConsoleProvider provider = new();
+
+		// Act
+		panel.ArrangeChildren();
+		panel.Render(provider);
+
+		// Assert
+		Assert.AreEqual(new Dimensions(40, 1), text.Dimensions);
+		Assert.AreEqual(expectedX, provider.WritesOf("hi").Single().Position.X);
+	}
+
+	/// <summary>
+	/// A horizontal stack gives each child the panel's full height and keeps the measured width on
+	/// the stacking axis, so the stacking offsets do not change (ktsu-dev/TUI#158).
+	/// </summary>
+	[TestMethod]
+	public void StackPanelHorizontalChildSpansThePanelHeight()
+	{
+		// Arrange
+		TextElement first = new("abc");
+		TextElement second = new("de");
+		StackPanel panel = new(Orientation.Horizontal) { Spacing = 1 };
+		panel.AddChild(first);
+		panel.AddChild(second);
+		panel.Dimensions = new Dimensions(20, 5);
+
+		// Act
+		panel.ArrangeChildren();
+
+		// Assert
+		Assert.AreEqual(new Dimensions(3, 5), first.Dimensions);
+		Assert.AreEqual(new Dimensions(2, 5), second.Dimensions);
+		Assert.AreEqual(new Position(4, 0), second.Position);
+	}
+
 	/// <summary>
 	/// Tests that a container that does not use the available width, such as BorderElement, measures
 	/// the same with or without one.

TUI.Test/UIApplicationResizeTests.cs

@@ -90,7 +90,7 @@ public void RenderAfterAResizeRelaysOutEveryContainerBeneathTheRoot()
 		UIApplication app = new(provider) { RootElement = root };
 		app.Render();
 		Assert.AreEqual(LaunchSize.WithoutPadding(root.Padding), panel.Dimensions, "The panel should start out filling the terminal, inside the border");
-		Assert.AreEqual(WideText.Length, leaf.Dimensions.Width, "The label should start out at its full width");
+		Assert.AreEqual(panel.Dimensions.Width, leaf.Dimensions.Width, "The label should start out spanning the panel");
 
 		// Act
 		provider.Dimensions = ResizedSize;

TUI/Elements/Layouts/StackPanel.cs

@@ -99,10 +99,11 @@ protected override void OnArrangeChildren()
 
 			if (Orientation == Orientation.Vertical)
 			{
-				// Vertical stacking
+				// Vertical stacking. The child spans the panel's width rather than its own measured
+				// width, so it has room to apply its own horizontal alignment (ktsu-dev/TUI#158).
 				child.Position = contentPosition.Offset(0, currentOffset);
 				child.Dimensions = new Dimensions(
-					Math.Min(childDimensions.Width, contentArea.Width),
+					contentArea.Width,
 					Math.Min(childDimensions.Height, contentArea.Height - currentOffset)
 				);
 
@@ -116,11 +117,11 @@ protected override void OnArrangeChildren()
 			}
 			else
 			{
-				// Horizontal stacking
+				// Horizontal stacking. The child spans the panel's height, matching the vertical case.
 				child.Position = contentPosition.Offset(currentOffset, 0);
 				child.Dimensions = new Dimensions(
 					Math.Min(childDimensions.Width, contentArea.Width - currentOffset),
-					Math.Min(childDimensions.Height, contentArea.Height)
+					contentArea.Height
 				);
 
 				currentOffset += child.Dimensions.Width + Spacing;