This three-line patch fixes all problems with `window-at' and
`coordinates-in-window-p', and as far as I can see it does not affect
any other part of Emacs.

Yet another way to fix the problem would be to change the window-edges
family of functions to return ugly floats instead of nice round
integers, but on the Lisp level I think it's better to have the
frame's origin inside the frame's border.  I like the above patch the
most.

> A patch that Richard sent me and that does this for `window-at'
> strangely enough seems to fail by making `window-at' return nil
> everywhere:

This is strange, as the two patches look to be equivalent.

> *** window.c    20 Jul 2004 16:52:30 -0400      1.471
> --- window.c    23 Jul 2004 20:05:29 -0400
> ***************
> *** 937,944 ****
>     CHECK_NUMBER_OR_FLOAT (y);
>
>     return window_from_coordinates (f,
> !                                 FRAME_PIXEL_X_FROM_CANON_X (f, x),
> !                                 FRAME_PIXEL_Y_FROM_CANON_Y (f, y),
>                                   0, 0, 0, 0);
>   }
>
> --- 937,946 ----
>     CHECK_NUMBER_OR_FLOAT (y);
>
>     return window_from_coordinates (f,
> !                                 (FRAME_PIXEL_X_FROM_CANON_X (f, x)
> !                                  + FRAME_INTERNAL_BORDER_WIDTH (f)),
> !                                 (FRAME_PIXEL_Y_FROM_CANON_Y (f, y)
> !                                  + FRAME_INTERNAL_BORDER_WIDTH (f)),
>                                   0, 0, 0, 0);
>   }

-- 
Károly