calculate usable window width and height.
regardless of the where point is in the buffer and where the window
is placed in the frame."
(let* ((wind (if (null window) (selected-window) window))
- (usable-width (1- (window-width wind))) ; 1- for cont. column
- (usable-height (1- (window-height wind))) ; 1- for mode line
(big-hairy-result (compute-motion
(window-start)
'(0 . 0)
pos
- (cons usable-width usable-height)
- usable-width
+ nil ; (window-width window-height)
+ nil ; window-width
(cons (window-hscroll)
0) ; why zero?
wind)))