Return non-nil if position POS is currently on the frame in WINDOW.
WINDOW must be a live window and defaults to the selected one.
Return nil if that position is scrolled vertically out of view. If a
character is only partially visible, nil is returned, unless the
optional argument PARTIALLY is non-nil. If POS is only out of view
because of horizontal scrolling, return non-nil. If POS is t, it
specifies the position of the last visible glyph in WINDOW. POS
defaults to point in WINDOW; WINDOW defaults to the selected window.
If POS is visible, return t if PARTIALLY is nil; if PARTIALLY is non-nil,
return value is a list of 2 or 6 elements (X Y [RTOP RBOT ROWH VPOS]),
where X and Y are the pixel coordinates relative to the top left corner
of the window. The remaining elements are omitted if the character after
POS is fully visible; otherwise, RTOP and RBOT are the number of pixels
off-window at the top and bottom of the row, ROWH is the height of the
display row, and VPOS is the row number (0-based) containing POS. (fn &optional POS WINDOW PARTIALLY)