Fix bug #11367 with assertion violation during vertical motion in egg.el.

 src/xdisp.c (pos_visible_p): If already at a newline from the
 display string before the 'while' loop, don't walk back the glyphs
 from it3.glyph_row.  Solves assertion violation when the display
 string begins with a newline (egg.el).

src/ChangeLog

+2012-04-29  Eli Zaretskii  <eliz@gnu.org>
+
+	* xdisp.c (pos_visible_p): If already at a newline from the
+	display string before the 'while' loop, don't walk back the glyphs
+	from it3.glyph_row.  Solves assertion violation when the display
+	string begins with a newline (egg.el).  (Bug#11367)
+
 2012-04-24  Chong Yidong  <cyd@gnu.org>
 
 	* xselect.c (x_convert_selection): Initialize a pointer (Bug#11315).

src/xdisp.c

@@ -1375,6 +1375,7 @@ pos_visible_p (struct window *w, EMACS_INT charpos, int *x, int *y,
 		  Lisp_Object startpos, endpos;
 		  EMACS_INT start, end;
 		  struct it it3;
+		  int it3_moved;
 		  /* Find the first and the last buffer positions
 		     covered by the display string.  */
@@ -1431,6 +1432,15 @@ pos_visible_p (struct window *w, EMACS_INT charpos, int *x, int *y,
 		     begins.  */
 		  start_display (&it3, w, top);
 		  move_it_to (&it3, -1, 0, top_y, -1, MOVE_TO_X | MOVE_TO_Y);
+		  /* If it3_moved stays zero after the 'while' loop
+		     below, that means we already were at a newline
+		     before the loop (e.g., the display string begins
+		     with a newline), so we don't need to (and cannot)
+		     inspect the glyphs of it3.glyph_row, because
+		     PRODUCE_GLYPHS will not produce anything for a
+		     newline, and thus it3.glyph_row stays at its
+		     stale content it got at top of the window.  */
+		  it3_moved = 0;
 		  /* Finally, advance the iterator until we hit the
 		     first display element whose character position is
 		     CHARPOS, or until the first newline from the
@@ -1442,6 +1452,7 @@ pos_visible_p (struct window *w, EMACS_INT charpos, int *x, int *y,
 		      if (IT_CHARPOS (it3) == charpos
 			  || ITERATOR_AT_END_OF_LINE_P (&it3))
 			break;
+		      it3_moved = 1;
 		      set_iterator_to_next (&it3, 0);
 		    }
 		  top_x = it3.current_x - it3.pixel_width;
@@ -1452,7 +1463,8 @@ pos_visible_p (struct window *w, EMACS_INT charpos, int *x, int *y,
 		     display string, move back over the glyphs
 		     produced from the string, until we find the
 		     rightmost glyph not from the string.  */
-		  if (IT_CHARPOS (it3) != charpos && EQ (it3.object, string))
+		  if (it3_moved
+		      && IT_CHARPOS (it3) != charpos && EQ (it3.object, string))
 		    {
 		      struct glyph *g = it3.glyph_row->glyphs[TEXT_AREA]
 					+ it3.glyph_row->used[TEXT_AREA];