// screen coordinates. Otherwise, an empty rectangle is returned.
// |pane_window| is the window pointer of the pane being dragged, |pt| is
// the mouse position, in client coordinates. |offset| describes the offset
// screen coordinates. Otherwise, an empty rectangle is returned.
// |pane_window| is the window pointer of the pane being dragged, |pt| is
// the mouse position, in client coordinates. |offset| describes the offset