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>
OnEvent handles the Scroll wheel in grid mode (mirrors UiText's sign), driving
the shared UiScrollable. The bound gutter scrollbar (next task) is the primary
scroll affordance; the wheel is the convenience path.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Grid mode now drives a shared UiScrollable from its content + clips whole rows
to the panel (cells offset by -ScrollY, Visible toggled by whole-row clip,
mirroring UiText). Fill mode (toolbar single cell) unchanged. LayoutCells made
internal for tests; RowCount helper added.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
