1) On Windows, when a modifier key, such as Shift is pressed, new DragMove event is generated. macOS should behave the same way too (I did not test myself).

2) On Linux, dragging simply ignores all key events (except esc) and updates modifier state only after moving mouse when holding a key down. Dropping seems to notice modifier keys.

To reproduce:

• Run attached application
• start dragging the circle and stop
• press shift, ctrl or alt
  -> nothing happens (no move event)

Instead it should probably behave like on other platforms.