Root cause of the "grid escapes the window when scrolled" bug: UiElement.
DrawSelfAndChildren runs ApplyAnchor on every child AFTER OnDraw, which captured
each cell's scroll-0 position and reset Top to it every frame, fighting
LayoutCells' scroll offset. Cells are laid out procedurally by the list, so they
must be exempt — AddItem now sets cell.Anchors = None, making LayoutCells the
sole authority. Regression test reproduces the anchor-pass interaction (the unit
tests missed it by calling LayoutCells in isolation, never the draw traversal).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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