Found at visual verification: an occupied UiItemSlot sits inside the Draggable
toolbar frame (UiNineSlicePanel.Draggable=true), so UiRoot.OnMouseDown's FindWindow
returned the frame and the window-move branch won — press+drag on a slot moved the
whole bar instead of picking up the item. The slot wasn't CapturesPointerDrag (that
path is for self-driven text-selection and suppresses the BeginDrag promotion), and
UiRoot had no path for "a drag-source inside a draggable window."
Fix: add UiElement.IsDragSource (virtual, default false); UiItemSlot overrides it to
`ItemId != 0` (occupancy-gated). UiRoot.OnMouseDown now prioritizes IsDragSource over
window-move — an OCCUPIED slot starts the item drag (promotes to BeginDrag on >3px),
an EMPTY slot falls through to the IA-12 whole-window-drag so the bar stays movable
by its empty cells / chrome. UiRoot stays item-agnostic (reads only the bool). This
REDUCES divergence (occupied cells now drag like retail) within IA-12's umbrella — no
new register row.
Regression tests reproduce the LIVE topology (slot inside a Draggable frame); the
earlier RootWithBoundSlot tests put the slot directly under the root, so they could
not catch it. Full suite 493 pass / 0 fail / 2 skip.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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