bug#12600: 24.2.50; linum-mode: line numbers in fringe do not refresh when resizing frame

[martin rudalics]

>> BTW, I meanwhile discovered that the update_mode_line field (with a
>> misleading comment IIUC) is the canonical approach to ask for
>> redisplaying a specific window.
>
> That's not my reading of the code.  The w->update_mode_line flag, if
> non-zero, forces redisplay of the mode line, and also of the menu bar
> and the tool bar, of that window.  Where did you see that its effect
> is to redisplay the whole window?

Because the doc-string for Fforce_window_update says "If optional arg
OBJECT is a window, force redisplay of that window only." but the code
does

      mark_window_display_accurate (object, 0);
      w->update_mode_line = 1;
      if (BUFFERP (w->buffer))
        XBUFFER (w->buffer)->prevent_redisplay_optimizations_p = 1;
      ++update_mode_lines;

and does not set windows_or_buffers_changed.  So apparently the above
(and not only setting w->update_mode_line to 1 as I concluded
incorrectly) are sufficient to force the redisplay of a single window
(including all windows showing the same buffer).

> force-window-update with a window as its argument just marks that one
> window's display "inaccurate", requests redisplay of its mode-line,
> and prevents optimizations of any other window displaying the same
> buffer.  Windows that don't display this buffer can still avoid some
> or all of redisplay through optimizations.

That's the case I had in mind.  So if I follow you correctly,
set_window_buffer could instead of doing

  windows_or_buffers_changed++;

call Fforce_window_update with the window whose buffer gets set as its
argument.

> I think resizing requests a thorough redisplay anyway,

Resizing a frame almost certainly does.  change_frame_size_1 does

  adjust_glyphs (f);
  calculate_costs (f);
  SET_FRAME_GARBAGED (f);
  f->resized_p = 1;

but none of these seem useful for Fwindow_end (are they useful anywhere
at all?), so ...

> setting windows_or_buffers_changed cannot hurt.

... I _had to_ set this in order to make the check in Fwindow_end fail.

martin