Return a list (FRAME X . Y) giving the current mouse frame and position.
The position is given in character cells, where (0, 0) is the
upper-left corner of the frame, X is the horizontal offset, and Y is
the vertical offset.
If Emacs is running on a mouseless terminal or hasn't been programmed
to read the mouse position, it returns the selected frame for FRAME
and nil for X and Y.
If `mouse-position-function' is non-nil, `mouse-position' calls it,
passing the normal return value to that function as an argument,
and returns whatever that function returns. (fn)