fix: invalidate each ancestor once instead of 2^depth times

TUI.Test/UIContainerBaseTests.cs

@@ -67,4 +67,54 @@ public void AMovedChildNoLongerInvalidatesItsPreviousContainer()
 
 		Assert.AreEqual(0, firstInvalidations);
 	}
+
+	/// <summary>
+	/// A change deep in the tree reaches the root once, not once per route at every level
+	/// (ktsu-dev/TUI#148)
+	/// </summary>
+	[TestMethod]
+	public void AChangeNestedManyLevelsDeepInvalidatesTheRootOnce()
+	{
+		StackPanel root = [];
+		StackPanel current = root;
+		for (int i = 0; i < 15; i++)
+		{
+			StackPanel next = [];
+			current.AddChild(next);
+			current = next;
+		}
+
+		TextElement leaf = new() { Text = "x" };
+		current.AddChild(leaf);
+
+		int count = 0;
+		root.Invalidated += (_, _) => count++;
+
+		leaf.Text = "y";
+
+		Assert.AreEqual(1, count);
+	}
+
+	/// <summary>
+	/// Each ancestor between the change and the root is invalidated exactly once
+	/// </summary>
+	[TestMethod]
+	public void AChangeInvalidatesEachAncestorOnce()
+	{
+		StackPanel root = [];
+		StackPanel middle = [];
+		TextElement leaf = new() { Text = "x" };
+		root.AddChild(middle);
+		middle.AddChild(leaf);
+
+		int rootCount = 0;
+		int middleCount = 0;
+		root.Invalidated += (_, _) => rootCount++;
+		middle.Invalidated += (_, _) => middleCount++;
+
+		leaf.Text = "y";
+
+		Assert.AreEqual(1, middleCount);
+		Assert.AreEqual(1, rootCount);
+	}
 }

TUI/Contracts/IUIElement.cs

@@ -67,8 +67,8 @@ public interface IUIElement
 	public Dimensions CalculateRequiredDimensions(int availableWidth) => CalculateRequiredDimensions();
 
 	/// <summary>
-	/// Marks the element as changed since its last draw, notifies its parent, and raises the
-	/// invalidated event
+	/// Marks the element as changed since its last draw and raises the invalidated event, which is
+	/// how the containing parent hears about the change
 	/// </summary>
 	/// <remarks>
 	/// Rendering redraws every visible element on every pass, so this does not decide whether an

TUI/Elements/UIElementBase.cs

@@ -103,10 +103,14 @@ public virtual void Render(IConsoleProvider provider)
 	public virtual Dimensions CalculateRequiredDimensions(int availableWidth) => OnCalculateRequiredDimensions(availableWidth);
 
 	/// <inheritdoc />
+	/// <remarks>
+	/// The parent hears about this through <see cref="Invalidated"/>, which every container
+	/// subscribes to when it adopts a child. Calling the parent directly as well would reach it by
+	/// two routes, doubling the count at every level (ktsu-dev/TUI#148).
+	/// </remarks>
 	public void Invalidate()
 	{
 		IsDirty = true;
-		Parent?.Invalidate();
 		Invalidated?.Invoke(this, EventArgs.Empty);
 	}