[macOS] Custom drag cursor is not applied in the border of drop area

Set a custom drag cursor with QDrag::setDragCursor(). Now, when dragging an item from forbidden area to drop area, the cursor is changed to the native copy cursor on the border of the drop area, and then it is correctly switched to the custom drag cursor. The area where the native copy cursor is shown is narrow and therefore this is not obvious to observe, but dragging slowly cross the border does the trick - at least an unusual blink should be relatively easy to see. Moving back from the drop area to the forbidden area works correctly.

The attached example demonstrates the problem. Drag an item from left hand side to right. The cursor changes when accessing drop area. Cross this boundary slowly from left to right - adding vertical movement (and patience) helps to find the exact point.