A change to a nested element invalidates the root 2^depth times, because each level passes Invalidate up both directly and through the Invalidated event

[matt-edmondson]

What's wrong

An invalidation travels up the tree by two routes at once:

• UIElementBase.Invalidate() (TUI/Elements/UIElementBase.cs) calls Parent?.Invalidate() and raises Invalidated.
• UIContainerBase subscribes child.Invalidated += OnChildInvalidated for every child it adds, and OnChildInvalidated calls Invalidate() again (TUI/Elements/UIContainerBase.cs:28, :153).

So every parent's Invalidate() runs twice for each child invalidation, and the count doubles at each level.

Repro

StackPanel root = new(); StackPanel cur = root;
for (int i = 0; i < 15; i++) { StackPanel n = new(); cur.AddChild(n); cur = n; }
TextElement leaf = new("x"); cur.AddChild(leaf);

int count = 0;
root.Invalidated += (_, _) => count++;
leaf.Text = "y";
// count == 65536 (2^16); expected 1

Why it matters

Every setter that invalidates, including Text, Position, Dimensions and IsVisible, goes through this. During arrange, every node in the tree has its Position/Dimensions set, so the cost of a relayout grows exponentially with depth. A typical 3–4-level UI already invalidates the root 8–16 times per change, and any handler on Invalidated fires that many times.

Suggested fix

Keep one propagation path:

• drop Parent?.Invalidate() from UIElementBase.Invalidate() and rely on the container's event subscription, or
• drop the OnChildInvalidated subscription and its unsubscribes in RemoveChild/ClearChildren.

Acceptance: a test that sets Text on a leaf nested N levels deep sees the root's Invalidated fire exactly once.

[matt-edmondson]

Triage

• Category: Bug (performance)
• Priority: High. Invalidation grows exponentially with tree depth (2^depth), and every layout pass sets Position and Dimensions on every node. A 4-level UI already invalidates the root 16 times per change, and deeper trees become unusable. The fix is to remove one line or one subscription.
• Area / suggested assignee: UIElementBase.Invalidate and UIContainerBase.OnChildInvalidated. Owner: @matt-edmondson
• Duplicates / in progress: none found. Open PR Give StackPanel children that no longer fit no space, so they stop rendering #147 (StackPanel arrange) is in the layout area but does not touch invalidation propagation.
• Next step: keep a single propagation path, then add a test asserting that the root fires once for a change N levels deep.

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