Truncating scroll runs that copy to where we copied to

[YAMAMOTO Mitsuharu <mituharu@math.s.chiba-u.ac.jp>]

I think that `scrolling_window' needs to truncate scroll runs that
copy to where we copied to; otherwise, `assign_row (to, from)' assigns
a previously disabled bogus row in the desired matrix when we have an
overlap in the copy destination.  Such truncation can also avoid
unnecessary copy in the actual graphics operation.

Could someone double-check the code below?

				     YAMAMOTO Mitsuharu
				mituharu@math.s.chiba-u.ac.jp

=== modified file 'src/dispnew.c'
*** src/dispnew.c	2011-11-19 08:39:42 +0000
--- src/dispnew.c	2011-11-20 06:44:25 +0000
***************
*** 4552,4562 ****
  	    rif->clear_window_mouse_face (w);
  	    rif->scroll_run_hook (w, r);
  
! 	    /* Invalidate runs that copy from where we copied to.  */
  	    for (j = i + 1; j < nruns; ++j)
  	      {
  		struct run *p = runs[j];
  
  		if ((p->current_y >= r->desired_y
  		     && p->current_y < r->desired_y + r->height)
  		    || (p->current_y + p->height >= r->desired_y
--- 4552,4594 ----
  	    rif->clear_window_mouse_face (w);
  	    rif->scroll_run_hook (w, r);
  
! 	    /* Truncate runs that copy to where we copied to, and
! 	       invalidate runs that copy from where we copied to.  */
  	    for (j = i + 1; j < nruns; ++j)
  	      {
  		struct run *p = runs[j];
  
+ 		if (p->nrows > 0
+ 		    && p->desired_vpos < r->desired_vpos + r->nrows
+ 		    && p->desired_vpos + p->nrows > r->desired_vpos)
+ 		  {
+ 		    if (p->desired_vpos < r->desired_vpos)
+ 		      {
+ 			p->nrows = r->desired_vpos - p->desired_vpos;
+ 			p->height = r->desired_y - p->desired_y;
+ 		      }
+ 		    else
+ 		      {
+ 			int nrows_copied = (r->desired_vpos + r->nrows
+ 					    - p->desired_vpos);
+ 
+ 			if (p->nrows <= nrows_copied)
+ 			  p->nrows = 0;
+ 			else
+ 			  {
+ 			    int height_copied = (r->desired_y + r->height
+ 						 - p->desired_y);
+ 
+ 			    p->current_vpos += nrows_copied;
+ 			    p->desired_vpos += nrows_copied;
+ 			    p->nrows -= nrows_copied;
+ 			    p->current_y += height_copied;
+ 			    p->desired_y += height_copied;
+ 			    p->height -= height_copied;
+ 			  }
+ 		      }
+ 		  }
+ 
  		if ((p->current_y >= r->desired_y
  		     && p->current_y < r->desired_y + r->height)
  		    || (p->current_y + p->height >= r->desired_y