Move window {nr}'s status line (i.e., the bottom border) by {offset} rows, as if being dragged by the mouse. {nr} can be a window number or window-ID. A positive {offset} moves down and a negative {offset} moves up. Moving a window's status line will change the height of the window and the height of other windows adjacent to the status line. The magnitude of movement may be smaller than specified (e.g., as a consequence of maintaining 'winminheight'). Returns TRUE if the window can be found and FALSE otherwise. Only works for the current tab page.

Can also be used as a method:

GetWinnr()->win_move_statusline(offset)