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Re: Redisplay bug

From: Kim F. Storm
Subject: Re: Redisplay bug
Date: 26 Aug 2002 01:10:51 +0200
User-agent: Gnus/5.09 (Gnus v5.9.0) Emacs/21.3.50

Richard Stallman <address@hidden> writes:

>     I found a specific part of Richard's change to xdisp.c which seems
>     to cause the redisplay problem, so I have reverted that change.
>     But now the redisplay again has a habit of leaving the cursor on
>     partially visible lines at the top or bottom of the window (which
>     I believe is what Richard tried to fix).  
> Of course.  Reverting that part of the change shows you found the
> right place, but it is not the right fix.
> Can you show me which change it was?

Here is the change I made.  Only one line of code with a large comment
on why I changed this.  Hope it is enough for you to work on.

Index: xdisp.c
RCS file: /cvs/emacs/src/xdisp.c,v
retrieving revision 1.776
retrieving revision 1.777
diff -c -r1.776 -r1.777
*** xdisp.c     9 Aug 2002 17:29:11 -0000       1.776
--- xdisp.c     22 Aug 2002 16:52:56 -0000      1.777
*** 10365,10371 ****
        if (!make_cursor_line_fully_visible (w))
!       goto try_to_scroll;
        debug_method_add (w, "forced window start");
--- 10365,10408 ----
        if (!make_cursor_line_fully_visible (w))
!       {
!         /* CVS rev. 1.761 had changed this to ``goto try_to_scroll''.
!            The intention of the fix -- AFAIU -- was to ensure that 
!            the cursor didn't end up on a partially visible last (or
!            first?) line when scrolling.
!            But that change causes havoc when scrolling backwards and
!            a partially visible first (or last?) line is present when
!            we reach the top of the buffer.  In effect, the text
!            already in the window is repeated (each line is appended
!            to the same or another lines in the window)...
!            I changed it back to ``goto need_larger_matrices'' which
!            in effect mean that we don't go through `try_scrolling'
!            when the cursor is already at the first line of the buffer,
!            and there is really only a few pixels [rather than lines]
!            to scroll backwards.  I guess move_it_by_lines etc. really
!            isn't the right device for doing that, ref. the code in
!            make_cursor_line_fully_visible which was also disabled by
!            CVS rev. 1.761.
!            But how do we know that we are already on the top line of
!            the window showing the first line in the buffer, so that
!            scrolling really wont help here?
!            I cannot find a simple fix for this (I tried various
!            approaches), but I prefer to an occasional partial line
!            rather than the visual messup, so I reverted this part of
!            the fix.
!            Someone will need to look into this when time allows.
!            -- 2002-08-22, Kim F. Storm  */
!         goto need_larger_matrices;
!       }
        debug_method_add (w, "forced window start");

Kim F. Storm <address@hidden> http://www.cua.dk

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