window-full-width-p

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

(window-full-width-p &optional WINDOW)

Return t if WINDOW is as wide as its containing frame.
More precisely, return t if and only if the total width of WINDOW
equals the total width of the root window of WINDOW's frame.
WINDOW must be a valid window and defaults to the selected one.