Leave a hidden child out of BorderElement's and the default container measure [patch]

TUI.Test/BorderElementRenderTests.cs

@@ -234,6 +234,32 @@ public void TitleWithALineBreakIsDrawnOnOneRow(string title)
 		Assert.AreEqual(0, provider.WritesOf(" ab cd ").Single().Position.Y);
 	}
 
+	/// <summary>
+	/// A border whose child is hidden measures as the bare border, so collapsing a section inside a
+	/// vertical StackPanel gives its rows back to the elements below (ktsu-dev/TUI#190).
+	/// </summary>
+	[TestMethod]
+	public void HiddenChildIsMeasuredAsAbsent()
+	{
+		// Arrange
+		BorderElement border = new() { Child = new TextElement("details line one\ndetails line two") { IsVisible = false } };
+		StackPanel panel = new(Orientation.Vertical);
+		panel.AddChild(border);
+		panel.AddChild(new TextElement("below"));
+		panel.Position = Position.Origin;
+		panel.Dimensions = new Dimensions(40, 20);
+		RecordingConsoleProvider provider = new();
+
+		// Act
+		panel.ArrangeChildren();
+		panel.Render(provider);
+
+		// Assert
+		Assert.AreEqual(new Dimensions(2, 2), border.CalculateRequiredDimensions());
+		Assert.AreEqual(new Dimensions(2, 2), border.CalculateRequiredDimensions(40));
+		Assert.AreEqual(2, provider.WritesOf("below").Single().Position.Y);
+	}
+
 	/// <summary>
 	/// A border sized by a layout is measured wide enough for its title, so a title longer than
 	/// the content is drawn in full rather than cut short or dropped (ktsu-dev/TUI#157).

TUI.Test/UIContainerBaseMeasureTests.cs

@@ -42,6 +42,20 @@ private static OriginContainer CreateContainer(Position position)
 		return container;
 	}
 
+	/// <summary>
+	/// A hidden child's extent is left out of the default measurement (ktsu-dev/TUI#190)
+	/// </summary>
+	[TestMethod]
+	public void DefaultMeasurementIgnoresHiddenChildren()
+	{
+		OriginContainer container = CreateContainer(Position.Origin);
+		container.AddChild(new TextElement { Text = "a much longer hidden line", IsVisible = false });
+		container.ArrangeChildren();
+
+		Assert.AreEqual(new Dimensions(5, 3), container.CalculateRequiredDimensions());
+		Assert.AreEqual(new Dimensions(5, 3), container.CalculateRequiredDimensions(80));
+	}
+
 	/// <summary>
 	/// The required size is the content plus one padding, not the screen position plus two paddings
 	/// </summary>

TUI/Elements/Primitives/BorderElement.cs

@@ -217,15 +217,17 @@ protected override void OnArrangeChildren()
 	// class adds to this result, so only the child's own size belongs here (ktsu-dev/TUI#132).
 	// A title is drawn as " title " between the corners, so the content area must be at least
 	// two wider than the title or a layout sizes the border too narrow to show it (ktsu-dev/TUI#157).
+	// A hidden child is measured as absent, so a collapsed section shrinks to the bare border
+	// (ktsu-dev/TUI#190).
 	protected override Dimensions OnCalculateRequiredDimensionsForChildren() =>
-		WithRoomForTitle(Children.Count == 0 ? Dimensions.Empty : Children.First().CalculateRequiredDimensions());
+		WithRoomForTitle(GetVisibleChildren().FirstOrDefault()?.CalculateRequiredDimensions() ?? Dimensions.Empty);
 
 	/// <inheritdoc />
 	// The base class has already taken the border off availableWidth, so the child is measured
 	// against the width it will be arranged into. Without this a word-wrapped child is measured
 	// unwrapped and given a single row (ktsu-dev/TUI#161).
 	protected override Dimensions OnCalculateRequiredDimensionsForChildren(int availableWidth) =>
-		WithRoomForTitle(Children.Count == 0 ? Dimensions.Empty : Children.First().CalculateRequiredDimensions(availableWidth));
+		WithRoomForTitle(GetVisibleChildren().FirstOrDefault()?.CalculateRequiredDimensions(availableWidth) ?? Dimensions.Empty);
 
 	private Dimensions WithRoomForTitle(Dimensions childDimensions)
 	{

TUI/Elements/UIContainerBase.cs

@@ -176,7 +176,8 @@ protected virtual Dimensions OnCalculateRequiredDimensionsForChildren()
 		int maxWidth = 0;
 		int maxHeight = 0;
 
-		foreach (IUIElement child in _children)
+		// A hidden child is not drawn, so it takes no room either (ktsu-dev/TUI#190)
+		foreach (IUIElement child in GetVisibleChildren())
 		{
 			Dimensions childDimensions = child.CalculateRequiredDimensions();
 			int childRight = Math.Max(0, child.Position.X - origin.X + childDimensions.Width);