Re: delete-overlay causes recentering

[Chong Yidong <cyd@stupidchicken.com>]

bojohan+news@dd.chalmers.se (Johan Bockgård) writes:

> Evaluate the code below. Then press f8, f9, f8, f9, ...
>
> (defvar ov nil)
>
> (defun foo ()
>   (interactive)
>   (set-window-start (selected-window) 834)
>   (goto-char 1237)
>   (setq ov (make-overlay 1237 1242))
>   (overlay-put ov 'face 'highlight))
>
> (defun bar ()
>   (interactive)
>   (delete-overlay ov))
>
> (global-set-key [f8] 'foo)
> (global-set-key [f9] 'bar)
>
> (progn
>   (switch-to-buffer "*foo*")
>   (dotimes (n 30)
>     (insert "Emacs is the extensible, customizable, "
>             "self-documenting real-time display editor. "))
>   (goto-char (point-min))
>   (insert " "))
>
> delete-overlay causes a recentering. (Actually, so do mouse clicks in
> this case.)

The behavior arose from the following change:

2006-04-20  Kim F. Storm  <storm@cua.dk>

	* xdisp.c (redisplay_window): Fix last change.

	* xdisp.c (redisplay_window): If current window start is not at the
	beginning of a line, select a new window start if buffer is modified
	and window start is in the modified region, but the first change is
	before window start.


*** emacs/src/xdisp.c	2006/04/13 01:21:48	1.1086
--- emacs/src/xdisp.c	2006/04/20 08:46:56	1.1088
***************
*** 12689,12696 ****
        /* IT may overshoot PT if text at PT is invisible.  */
        else if (IT_CHARPOS (it) > PT && CHARPOS (startp) <= PT)
  	w->force_start = Qt;
- 
- 
      }
  
    /* Handle case where place to start displaying has been specified,
--- 12689,12694 ----
***************
*** 12860,12865 ****
--- 12858,12892 ----
  	       || (XFASTINT (w->last_modified) >= MODIFF
  		   && XFASTINT (w->last_overlay_modified) >= OVERLAY_MODIFF)))
      {
+ 
+       /* If first window line is a continuation line, and window start
+ 	 is inside the modified region, but the first change is before
+ 	 current window start, we must select a new window start.*/
+       if (NILP (w->start_at_line_beg))
+ 	{
+ 	  /* Make sure beg_unchanged and end_unchanged are up to date.
+ 	     Do it only if buffer has really changed.  This may or may
+ 	     not have been done by try_window_id (see which) already. */
+ 	  if (MODIFF > SAVE_MODIFF
+ 	      /* This seems to happen sometimes after saving a buffer.  */
+ 	      || BEG_UNCHANGED + END_UNCHANGED > Z_BYTE)
+ 	    {
+ 	      if (GPT - BEG < BEG_UNCHANGED)
+ 		BEG_UNCHANGED = GPT - BEG;
+ 	      if (Z - GPT < END_UNCHANGED)
+ 		END_UNCHANGED = Z - GPT;
+ 	    }
+ 
+ 	  if (CHARPOS (startp) > BEG + BEG_UNCHANGED
+ 	      && CHARPOS (startp) <= Z - END_UNCHANGED)
+ 	    {
+ 	      /* There doesn't seems to be a simple way to find a new
+ 		 window start that is near the old window start, so
+ 		 we just recenter.  */
+ 	      goto recenter;
+ 	    }
+ 	}
+ 
  #if GLYPH_DEBUG
        debug_method_add (w, "same window start");
  #endif