Return non-nil if COORDINATES are in WINDOW.
WINDOW must be a live window and defaults to the selected one.
COORDINATES is a cons of the form (X . Y), X and Y being distances
measured in characters from the upper-left corner of the frame.
(0 . 0) denotes the character in the upper left corner of the
If COORDINATES are in the text portion of WINDOW,
the coordinates relative to the window are returned.
If they are in the mode line of WINDOW, `mode-line' is returned.
If they are in the top mode line of WINDOW, `header-line' is returned.
If they are in the left fringe of WINDOW, `left-fringe' is returned.
If they are in the right fringe of WINDOW, `right-fringe' is returned.
If they are on the border between WINDOW and its right sibling,
`vertical-line' is returned.
If they are in the windows's left or right marginal areas, `left-margin'
or `right-margin' is returned. (fn COORDINATES WINDOW)