(try_window_reusing_current_matrix)

<new start <= old start>:  Disable rows in the current matrix
which are below the window after scrolling.

src/ChangeLog

 2000-12-29  Gerd Moellmann  <gerd@gnu.org>
 
+	* dispnew.c (check_current_matrix_flags) [GLYPH_DEBUG]: New
+	function.
+	(update_window) [GLYPH_DEBUG]: Call it.
+	(scrolling_window): Prevent including current rows which are below
+	what's displayed in the window.
+
+	* xdisp.c (try_window_reusing_current_matrix)
+	<new start <= old start>:  Disable rows in the current matrix
+	which are below the window after scrolling.
+	
 	* xdisp.c (move_it_by_lines): Don't do optimizations if NEED_Y_P
 	is zero.  It's not worth the complexity.
 	(invisible_text_between_p): Put in #if 0 because unused.

src/xdisp.c

@@ -10185,10 +10185,10 @@ try_window_reusing_current_matrix (w)
 			 MATRIX_ROW_VPOS (bottom_row, w->current_matrix),
 			 nrows_scrolled);
 	  
-	  /* Disable lines not reused.  */
+	  /* Disable lines that must be updated.  */
 	  for (i = 0; i < it.vpos; ++i)
 	    (start_row + i)->enabled_p = 0;
-	  
+
 	  /* Re-compute Y positions.  */
 	  min_y = WINDOW_DISPLAY_HEADER_LINE_HEIGHT (w);
 	  max_y = it.last_visible_y;
@@ -10213,6 +10213,11 @@ try_window_reusing_current_matrix (w)
 	      if (MATRIX_ROW_BOTTOM_Y (row) >= it.last_visible_y)
 		break;
 	    }
+	  
+	  /* Disable lines in the current matrix which are now
+	     below the window.  */
+	  for (; row < bottom_row; ++row)
+	    row->enabled_p = 0;
 	}
 
       /* Update window_end_pos etc.; last_reused_text_row is the last