Currently you can only call setdropAction() in dragMoveEvent() to change the drag cursor while dragging.
But you can not do anything to change the dropAction (and the drag cursor) if while dragging you stop moving the mouse and change the modifier key.

Step to reproduces with the sample code:
1. dragging the button inside its window.
2. stop moving the mouse. (this step is very important)
3. change the modifier key between Shift/Ctrl/...

• Expected behavior:
  The drag cursor changes automatically or manually like it does if you do the extra step below:
  4. Move the mouse some px.

• Current behavior:
  No way to change the drag cursor if you don't move the mouse after the step 3.