> (fringe-mode 0)
 > C-x 3 C-x 2 C-x o C-x o
 > (we're now in the large window on the right, with two small windows in
 > the left half of the frame).
 >
 > Insert enough newlines to be vertically just above the horizontal
 > divider on the left.
 >
 > M-: (posn-at-x-y 0 (cdr (posn-x-y (posn-at-point)))) RET
 >
 > Insert newline, now we're next to the divider line.
 >
 > M-: (posn-at-x-y 0 (cdr (posn-x-y (posn-at-point)))) RET
 >
 > --> wrong window (the window in the upper left) is given in the structure.
 > Instead of 0, the x value can be up to 6 ((frame-char-width) reports 7).

Can you try the attached patch?

martin