The problem was caused by a missing call to `WindowContext::notify` when
moving the mouse. Actually, we *did* notify as part of a `MouseMoveEvent`
listener registered in `Interactivity` but that code path was never
exercised because we were clearing the `pending_mouse_down`.
This commit fixes the issue by automatically redrawing the window in gpui
when there is an active drag and the mouse moves.
Antonio Scandurra
00927db711