Re: vertical-motion bug

[Richard Stallman]

      M-<
      M-: (insert (propertize "a" 'display "a\nb\nc\n")) RET
      M-<
      C-n (or M-: (vertical-motion 1))

    Result: point moves down three lines.  The expected behavior, based on
    the `vertical-motion' docstring, is to move just one line.

There is no way to move down just one line, since no buffer
position corresponds to that screen position.  Emacs can either
move three lines or not move.  So I think its actual behavior
is the best possible thing it could do.