UiMarkupList.OnDraw/DrawColumns re-ran their "keep the selected row
visible" clamp on EVERY draw call, not only when SelectedIndexSource
actually changed. A real bound list (MossTank's Advanced Options,
selected="{SelectedAdvancedOptionIndex}") holds a stable selected index
while the user operates the scrollbar, so the very next frame's OnDraw
saw that (unchanged) row now outside the just-scrolled view and yanked
_topRow straight back to it — undoing the arrow click or thumb drag
before it was ever visible. This is exactly the "sometimes works"
symptom: a scroll that happens to keep the selected row in view
survives; one that moves it out of view gets reverted on the next
frame. Gate the clamp on an observed change in the selected index
(int.MinValue sentinel so the first frame still reveals it), so a
stable selection no longer fights user-driven scrolling while a
genuinely new selection is still auto-revealed once.
New tests build a real nested Panel > Group > List tree at non-zero
offsets and drive it exclusively through UiRoot's public mouse entry
points, ruling out a coordinate-frame bug in the hit-test/dispatch
chain as well as proving the fix for both single-column and
<column>-mode lists (arrow click and thumb drag).
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
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