fix: arrange the tree on every render pass, not only on a resize

CLAUDE.md

@@ -54,6 +54,8 @@ Provider Abstraction (SpectreConsoleProvider)
    loop wakes on a timer (`UIApplication.ResizePollInterval`) as well as on input, because a
    resize delivers no keypress to wake it — each tick compares the size and draws only when it
    changed
+   The tree is arranged on every pass, not only on a resize: showing an element, changing text, or
+   a size the host assigned to the root all change layout without one (ktsu-dev/TUI#133)
 5. `UIContainerBase.Render()` renders itself then all visible children
 
 ## Code Patterns

TUI.Test/UIApplicationLayoutTests.cs

@@ -0,0 +1,90 @@
+// Copyright (c) 2023-2026 ktsu-dev contributors
+
+namespace ktsu.TUI.Test;
+
+using ktsu.TUI.Core.Elements.Layouts;
+using ktsu.TUI.Core.Elements.Primitives;
+using ktsu.TUI.Core.Models;
+using ktsu.TUI.Core.Services;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+/// <summary>
+/// Tests that <see cref="UIApplication"/> lays the tree out on every render pass.
+/// </summary>
+/// <remarks>
+/// Layout used to run only when the root had no size yet or the terminal was resized, so a change
+/// to the tree after the first frame kept its stale geometry (ktsu-dev/TUI#133).
+/// </remarks>
+[TestClass]
+public sealed class UIApplicationLayoutTests
+{
+	/// <summary>
+	/// An element made visible after the first frame must be laid out and drawn on the next one.
+	/// It used to keep its default position and empty size, so it was never drawn.
+	/// </summary>
+	[TestMethod]
+	public void RenderAfterShowingAHiddenElementDrawsIt()
+	{
+		// Arrange
+		RecordingConsoleProvider provider = new();
+		TextElement a = new("a");
+		TextElement b = new("b") { IsVisible = false };
+		StackPanel root = [a, b];
+		UIApplication app = new(provider) { RootElement = root };
+		app.Render();
+
+		// Act
+		b.IsVisible = true;
+		app.Render();
+
+		// Assert
+		Assert.Contains(
+			w => w.Text == "b" && w.Position == new Position(0, 1),
+			provider.Writes,
+			"The newly shown element should be drawn on the row below its sibling");
+	}
+
+	/// <summary>
+	/// A root the host sized itself must still have its children laid out. The arrange used to be
+	/// skipped because the root already had a size, so its children had none and nothing was drawn.
+	/// </summary>
+	[TestMethod]
+	public void RenderWithAHostAssignedRootSizeDrawsItsChildren()
+	{
+		// Arrange
+		RecordingConsoleProvider provider = new();
+		StackPanel root = [new TextElement("abc")];
+		root.Dimensions = new Dimensions(20, 5);
+		UIApplication app = new(provider) { RootElement = root };
+
+		// Act
+		app.Render();
+
+		// Assert
+		Assert.Contains(w => w.Text == "abc", provider.Writes, "The child of a host-sized root should be drawn");
+		Assert.AreEqual(new Dimensions(20, 5), root.Dimensions, "The host-assigned size should be kept");
+	}
+
+	/// <summary>
+	/// Text that grows after the first frame must push its siblings along on the next one.
+	/// </summary>
+	[TestMethod]
+	public void RenderAfterTextGrowsMovesTheFollowingSibling()
+	{
+		// Arrange
+		RecordingConsoleProvider provider = new();
+		TextElement first = new("ab");
+		TextElement second = new("cd");
+		StackPanel root = new(Orientation.Horizontal) { first, second };
+		UIApplication app = new(provider) { RootElement = root };
+		app.Render();
+		int before = second.Position.X;
+
+		// Act
+		first.Text = "abcdef";
+		app.Render();
+
+		// Assert
+		Assert.AreEqual(before + 4, second.Position.X, "The sibling should move right by the width the text gained");
+	}
+}

TUI/Services/UIApplication.cs

@@ -130,7 +130,7 @@
 		}
 
 		IsRunning = true;
 		_cancellationTokenSource = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
 		IDisposable? interruptRegistration = null;
 
 		try
@@ -270,7 +270,7 @@
 	}
 
 	/// <summary>
-	/// Brings the root element's size in line with the terminal, re-arranging the tree when it changes
+	/// Brings the root element's size in line with the terminal, then re-arranges the tree
 	/// </summary>
 	/// <param name="root">The root element to size</param>
 	/// <remarks>
@@ -287,20 +287,21 @@
 		// Adopt the terminal size when the root has none of its own, and again whenever the
 		// terminal is resized. In between, a size the host assigned to the root is left alone — a
 		// resize is the one thing that overrides it, since the old size no longer fits the window.
-		if (!resized && !root.Dimensions.IsEmpty)
+		if (resized || root.Dimensions.IsEmpty)
 		{
-			return;
-		}
+			if (resized && _logger != null)
+			{
+				LogConsoleResized(_logger, console.Width, console.Height, null);
+			}
 
-		if (resized && _logger != null)
-		{
-			LogConsoleResized(_logger, console.Width, console.Height, null);
+			root.Dimensions = console;
 		}
 
-		root.Dimensions = console;
-
-		// Assigning Dimensions only invalidates; it does not re-run layout. Walk the tree so every
-		// container re-arranges inside its new size, not just the root.
+		// Arrange on every pass, not only when the size changed. Showing an element, changing
+		// text, or a size the host assigned all change the layout without a resize, and skipping
+		// the arrange left them drawn with stale geometry (ktsu-dev/TUI#133). The pass already
+		// clears and redraws everything, so an arrange is cheap beside it. Assigning Dimensions
+		// only invalidates, so the tree is walked to re-arrange every container, not just the root.
 		ArrangeTree(root);
 	}
 
@@ -337,7 +338,7 @@
 		RootElement != null && ConsoleProvider.Dimensions != _observedConsoleDimensions;
 
 	/// <inheritdoc />
 	public async Task ProcessInputAsync(CancellationToken cancellationToken = default)
 	{
 		if (_logger != null)
 		{
@@ -415,7 +416,7 @@
 
 				if (!handled)
 				{
 					if (_logger != null)
 					{
 						LogInputNotHandled(_logger, null);
 					}
@@ -456,12 +457,12 @@
 
 				// Continue processing for recoverable errors
 			}
 			catch (OutOfMemoryException)
 			{
 				// Critical error - rethrow
 				throw;
 			}
 			catch (StackOverflowException)
 			{
 				// Critical error - rethrow
 				throw;