bug#44284: 27.1; with some Unicode font, scrolling upward with the mouse wheel actually scrolls downward when the cursor needs repositioning

[Vincent Lefevre]

What happens is the following. In xdisp.c, fucntion redisplay_window:

  /* Handle case where text has not changed, only point, and it has
     not moved off the frame, and we are not retrying after hscroll.
     (current_matrix_up_to_date_p is true when retrying.)  */
  if (current_matrix_up_to_date_p
      && (rc = try_cursor_movement (window, startp, &temp_scroll_step),
          rc != CURSOR_MOVEMENT_CANNOT_BE_USED))

rc is normally 0, but when the last line is reached, rc = 2
(CURSOR_MOVEMENT_MUST_SCROLL).

So I assume that the bug occurs somewhere in try_cursor_movement.

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Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
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Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)