adjust-window-trailing-edge is a compiled Lisp function in `window.el'.

(adjust-window-trailing-edge WINDOW DELTA &optional HORIZONTAL PIXELWISE)

Move WINDOW's bottom edge by DELTA lines.
Optional argument HORIZONTAL non-nil means move WINDOW's right
edge by DELTA columns. WINDOW must be a valid window and
defaults to the selected one.

Optional argument PIXELWISE non-nil means interpret DELTA as
number of pixels.

If DELTA is greater than zero, move the edge downwards or to the
right. If DELTA is less than zero, move the edge upwards or to
the left. If the edge can't be moved by DELTA lines or columns,
move it as far as possible in the desired direction.