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Re: Old redisplay bug in xdisp.c, question about change

From: Jan D.
Subject: Re: Old redisplay bug in xdisp.c, question about change
Date: Tue, 25 Mar 2003 20:35:33 +0100 (CET)

> This particular change was part of a group of changes made to prevent or
> reduce multiple unnecessary runs of menu-bar-update-hook on things like
> scrolls, tooltips, mouse-1 with mouse-sel-mode, C-s, C-x 2 or menu-bar
> clicks.  (The performance impact is so bad that I use 21.1.90 + these
> changes for all my editing, rather than 21.2 or 21.3 which do not have
> them.)  The second part of this particular change made the code match
> the comment; at the time Gerd said this code dated from at least 1994.
> It would be a shame if this change was simply reverted as the particular
> problem (multiple unnecessary runs of menu-bar-update-hook on scroll)
> will presumably resurface.  Does the fact that you don't always see a
> redisplay problem suggest that the bug lies elsewhere?

It the if-statement in update_tool_bar that is entered if w->update_mode_line
is Qt:

      /* If the user has switched buffers or windows, we need to
         recompute to reflect the new bindings.  But we'll
         recompute when update_mode_lines is set too; that means
         that people can use force-mode-line-update to request
         that the menu bar be recomputed.  The adverse effect on
         the rest of the redisplay algorithm is about the same as
         windows_or_buffers_changed anyway.  */
      if (windows_or_buffers_changed
          || !NILP (w->update_mode_line)
          || ((BUF_SAVE_MODIFF (XBUFFER (w->buffer))
               < BUF_MODIFF (XBUFFER (w->buffer)))
              != !NILP (w->last_had_star))
          || ((!NILP (Vtransient_mark_mode)
               && !NILP (XBUFFER (w->buffer)->mark_active))
              != !NILP (w->region_showing)))

The other parts are false, but I really don't know what they all mean.
windows_or_buffers_changed is 0 when I switch Info nodes.  In effect,
the buffer or the window has not changed, but its contents, active
and inactive menu and tool bar items have.

        Jan D.

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