window-resizable-p

window-resizable-p is a compiled Lisp function in `window.el'.

(window-resizable-p WINDOW DELTA &optional HORIZONTAL IGNORE PIXELWISE)

Return t if WINDOW can be resized vertically by DELTA lines.
WINDOW must be a valid window and defaults to the selected one.
For the meaning of the arguments of this function see the
doc-string of `window-resizable'.