bug#42406: Mouse-wheel scrolling can be flickering

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: alan@idiocy.org,  konrad.podczeck@univie.ac.at,  42406@debbugs.gnu.org
> Date: Fri, 18 Dec 2020 11:22:40 -0500
> 
> To be clear: I have no intention to push this to `emacs-27`, but
> I can't see any good reason not to push it to master (after fixing its
> FIXME, obviously).

I thought I already explained why I'm not interested in such "fixes".
I'd welcome a thorough redesign of the flags and introduction of new
flags to allow us to redraw specific parts like frame titles and menu
bars (or new values of existing flags to the same effect).  Then it
will be worth our while to risk breaking some use cases.  But I don't
want to increase the existing mess by lumping more and more logic on
top of what we already have, because that brings the same risks
without any benefits.  Sorry.

Oh, and your question about where the change in mode-line dimensions
is handled? it's here:

      display_mode_lines (w);

      /* If mode line height has changed, arrange for a thorough
         immediate redisplay using the correct mode line height.  */
      if (window_wants_mode_line (w)
          && CURRENT_MODE_LINE_HEIGHT (w) != DESIRED_MODE_LINE_HEIGHT (w))
        {
          f->fonts_changed = true;
          w->mode_line_height = -1;
          MATRIX_MODE_LINE_ROW (w->current_matrix)->height
            = DESIRED_MODE_LINE_HEIGHT (w);
        }
      [...]
      if (f->fonts_changed)
        goto need_larger_matrices;