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 pull request fixes the issue by automatically redrawing the window
in gpui when there is an active drag and the mouse moves.

Release Notes:

- N/A