Visual gate 2 (user): the "Slots" toggle caption was visible but unclickable.
Root cause (UiRoot.OnMouseDown/OnMouseUp): a left-press on a non-drag-source
widget inside a whole-window-Draggable frame (the inventory window's IA-12
drag) set _windowDragTarget; OnMouseUp then early-returned before emitting the
Click. So the paperdoll Slots button (the first plain button inside the
draggable inventory frame) never received its click. Chat/toolbar buttons
escape this — their frames aren't whole-window-draggable.
Fix (toolkit, root cause not band-aid): add UiElement.HandlesClick (a virtual
opt-out parallel to IsDragSource); UiButton overrides it true; OnMouseDown
routes a HandlesClick press to the widget (like CapturesPointerDrag) instead of
the window-drag, so OnMouseUp emits the Click. 2 regression tests lock it
(HandlesClick widget in a Draggable frame emits Click; a plain one doesn't).
Build + full App suite green (596, +2).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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