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).

2 files changed, 20 insertions, 1 deletions

diff --git a/src/ChangeLog b/src/ChangeLog
index 8c80a24b580..e2eb95f60d0 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,3 +1,10 @@
+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).
diff --git a/src/xdisp.c b/src/xdisp.c
index e53d3a57cd6..cc75d386f0d 100644
--- a/src/xdisp.c
+++ b/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];