Fix bug #17905 with display of point in partially visible line at end of window.

 src/xdisp.c (redisplay_window): If redisplay of a window ends up
 with point in a partially visible line at end of the window, make
 sure the amended position of point actually has smaller Y
 coordinate; if not, give up and scroll the display.
 src/window.c (window_scroll_pixel_based): When point ends up at the
 last fully visible line, don't let move_it_to stop at the left
 edge of the line and dupe us into thinking point is inside the
 scroll margin.

src/ChangeLog

 2014-07-04  Eli Zaretskii  <eliz@gnu.org>
 
+	* xdisp.c (redisplay_window): If redisplay of a window ends up
+	with point in a partially visible line at end of the window, make
+	sure the amended position of point actually has smaller Y
+	coordinate; if not, give up and scroll the display.  (Bug#17905)
+
+	* window.c (window_scroll_pixel_based): When point ends up at the
+	last fully visible line, don't let move_it_to stop at the left
+	edge of the line and dupe us into thinking point is inside the
+	scroll margin.
+
 	* w32.c (network_interface_info): Make sure the argument is a
 	Lisp string.
 

src/window.c

@@ -5167,6 +5167,32 @@ window_scroll_pixel_based (Lisp_Object window, int n, bool whole, int noerror)
       charpos = IT_CHARPOS (it);
       bytepos = IT_BYTEPOS (it);
 
+      /* If PT is in the screen line at the last fully visible line,
+	 move_it_to will stop at X = 0 in that line, because the
+	 required Y coordinate is reached there.  See if we can get to
+	 PT without descending lower in Y, and if we can, it means we
+	 reached PT before the scroll margin.  */
+      if (charpos != PT)
+	{
+	  struct it it2;
+	  void *it_data;
+
+	  it2 = it;
+	  it_data = bidi_shelve_cache ();
+	  move_it_to (&it, PT, -1, -1, -1, MOVE_TO_POS);
+	  if (IT_CHARPOS (it) == PT && it.current_y == it2.current_y)
+	    {
+	      charpos = IT_CHARPOS (it);
+	      bytepos = IT_BYTEPOS (it);
+	      bidi_unshelve_cache (it_data, 1);
+	    }
+	  else
+	    {
+	      it = it2;
+	      bidi_unshelve_cache (it_data, 0);
+	    }
+	}
+
       /* See if point is on a partially visible line at the end.  */
       if (it.what == IT_EOB)
 	partial_p = it.current_y + it.ascent + it.descent > it.last_visible_y;

src/xdisp.c

@@ -16106,6 +16106,18 @@ redisplay_window (Lisp_Object window, bool just_this_one_p)
 	  /* Point does appear, but on a line partly visible at end of window.
 	     Move it back to a fully-visible line.  */
 	  new_vpos = window_box_height (w);
+	  /* But if window_box_height suggests a Y coordinate that is
+	     not less than we already have, that line will clearly not
+	     be fully visible, so give up and scroll the display.
+	     This can happen when the default face uses a font whose
+	     dimensions are different from the frame's default
+	     font.  */
+	  if (new_vpos >= w->cursor.y)
+	    {
+	      w->cursor.vpos = -1;
+	      clear_glyph_matrix (w->desired_matrix);
+	      goto try_to_scroll;
+	    }
 	}
       else if (w->cursor.vpos >= 0)
 	{