Move window {nr}'s vertical separator (i.e., the right border) by {offset} columns, as if being dragged by the mouse. {nr} can be a window number or window-ID. A positive {offset} moves right and a negative {offset} moves left. Moving a window's vertical separator will change the width of the window and the width of other windows adjacent to the vertical separator. The magnitude of movement may be smaller than specified (e.g., as a consequence of maintaining 'winminwidth'). Returns TRUE if the window can be found and FALSE otherwise. This will fail for the rightmost window and a full-width window, since it has no separator on the right. Only works for the current tab page.

Can also be used as a method:

GetWinnr()->win_move_separator(offset)