From 7b9891890fe69b81879768cabf522431153a3115 Mon Sep 17 00:00:00 2001
From: Matthew Edmondson <matthew.edmondson@gmail.com>
Date: Tue, 29 Sep 2026 12:33:11 +0000
Subject: [PATCH] fix: measure a container's children from its content origin
 [patch]

The default OnCalculateRequiredDimensionsForChildren measured each child's
extent from screen column/row 0, so a custom container's required size
included its own position and its padding, which OnCalculateRequiredDimensions
then added again. A 5x3 container at (10,5) measured 16x9. Children are now
measured relative to GetContentPosition().

Fixes ktsu-dev/TUI#145

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VCL6GX9wshgqiBeq6z98ZH
---
 TUI.Test/UIContainerBaseMeasureTests.cs | 67 +++++++++++++++++++++++++
 TUI/Elements/UIContainerBase.cs         |  7 ++-
 2 files changed, 72 insertions(+), 2 deletions(-)
 create mode 100644 TUI.Test/UIContainerBaseMeasureTests.cs

diff --git a/TUI.Test/UIContainerBaseMeasureTests.cs b/TUI.Test/UIContainerBaseMeasureTests.cs
new file mode 100644
index 0000000..70cb036
--- /dev/null
+++ b/TUI.Test/UIContainerBaseMeasureTests.cs
@@ -0,0 +1,67 @@
+// Copyright (c) 2023-2026 ktsu-dev contributors
+
+namespace ktsu.TUI.Test;
+
+using ktsu.TUI.Core.Contracts;
+using ktsu.TUI.Core.Elements;
+using ktsu.TUI.Core.Elements.Primitives;
+using ktsu.TUI.Core.Models;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+/// <summary>
+/// Tests for the default measurement a custom container inherits from UIContainerBase
+/// </summary>
+[TestClass]
+public sealed class UIContainerBaseMeasureTests
+{
+	/// <summary>
+	/// A container that places every child at its content origin and keeps the default measurement
+	/// </summary>
+	private sealed class OriginContainer : UIContainerBase
+	{
+		protected override void OnRender(IConsoleProvider provider) { }
+
+		protected override void OnArrangeChildren()
+		{
+			foreach (IUIElement child in GetVisibleChildren())
+			{
+				child.Position = GetContentPosition();
+			}
+		}
+	}
+
+	private static OriginContainer CreateContainer(Position position)
+	{
+		OriginContainer container = new()
+		{
+			Position = position,
+			Padding = Padding.Uniform(1),
+		};
+		container.AddChild(new TextElement { Text = "abc" });
+		container.ArrangeChildren();
+		return container;
+	}
+
+	/// <summary>
+	/// The required size is the content plus one padding, not the screen position plus two paddings
+	/// </summary>
+	[TestMethod]
+	public void DefaultMeasurementIsContentPlusPadding()
+	{
+		OriginContainer container = CreateContainer(new Position(10, 5));
+
+		Assert.AreEqual(new Dimensions(5, 3), container.CalculateRequiredDimensions());
+	}
+
+	/// <summary>
+	/// Moving the container on screen does not change how large it asks to be
+	/// </summary>
+	[TestMethod]
+	public void DefaultMeasurementDoesNotDependOnPosition()
+	{
+		Dimensions atOrigin = CreateContainer(Position.Origin).CalculateRequiredDimensions();
+		Dimensions elsewhere = CreateContainer(new Position(40, 12)).CalculateRequiredDimensions();
+
+		Assert.AreEqual(atOrigin, elsewhere);
+	}
+}
diff --git a/TUI/Elements/UIContainerBase.cs b/TUI/Elements/UIContainerBase.cs
index ed7e5a7..306c05d 100644
--- a/TUI/Elements/UIContainerBase.cs
+++ b/TUI/Elements/UIContainerBase.cs
@@ -132,14 +132,17 @@ protected virtual Dimensions OnCalculateRequiredDimensionsForChildren()
 			return Dimensions.Empty;
 		}
 
+		// Children sit at absolute screen positions, so measure them from the content origin; the
+		// container's own position and padding are not part of the content's size
+		Position origin = GetContentPosition();
 		int maxWidth = 0;
 		int maxHeight = 0;
 
 		foreach (IUIElement child in _children)
 		{
 			Dimensions childDimensions = child.CalculateRequiredDimensions();
-			int childRight = child.Position.X + childDimensions.Width;
-			int childBottom = child.Position.Y + childDimensions.Height;
+			int childRight = Math.Max(0, child.Position.X - origin.X + childDimensions.Width);
+			int childBottom = Math.Max(0, child.Position.Y - origin.Y + childDimensions.Height);
 
 			maxWidth = Math.Max(maxWidth, childRight);
 			maxHeight = Math.Max(maxHeight, childBottom);