buttonImGuiMouseButton index: 0 left, 1 right, 2 middle.
lock_thresholdMinimum drag distance in pixels before dragging is recognized; -1 uses io.MouseDragThreshold.
Copied {x: dx, y: dy} screen-pixel displacement from the drag start; zero until the drag threshold is passed.
return the delta from the initial clicking position while the mouse button is pressed or was just released.
The result stays zero until the mouse passes the distance threshold at least once. If lock_threshold < 0.0, use io.MouseDragThreshold, where io is the field view returned by ImGui.viewImGuiIO(ioHandle).
Get the mouse displacement since dragging began.