Adjust point to move it off zero-width characters

[Eli Zaretskii]

Is the patch below a good idea?  If so, is it okay to install it on
the trunk?  It will allow us to set the glyphless-char-display of LRM,
RLM, and the other directional control characters to zero-width and
thus make them invisible without having the cursor get "stuck" on
them and without using invisible or intangible text properties.

=== modified file 'src/dispextern.h'
--- src/dispextern.h    2011-08-05 11:04:44 +0000
+++ src/dispextern.h    2011-08-06 14:39:22 +0000
@@ -3098,6 +3098,9 @@ extern int cursor_in_mouse_face_p (struc
 extern void tty_draw_row_with_mouse_face (struct window *, struct glyph_row *,
                                          int, int, enum draw_glyphs_face);
 
+extern Lisp_Object Qzero_width;
+extern Lisp_Object glyphless_char_display_method (int, struct frame *);
+
 /* Flags passed to try_window.  */
 #define TRY_WINDOW_CHECK_MARGINS       (1 << 0)
 #define TRY_WINDOW_IGNORE_FONTS_CHANGE (1 << 1)
@@ -3113,6 +3116,7 @@ void compute_fringe_widths (struct frame
 void w32_init_fringe (struct redisplay_interface *);
 void w32_reset_fringes (void);
 #endif
+
 /* Defined in image.c */
 
 #ifdef HAVE_WINDOW_SYSTEM

=== modified file 'src/keyboard.c'
--- src/keyboard.c      2011-08-04 17:04:39 +0000
+++ src/keyboard.c      2011-08-06 14:39:31 +0000
@@ -1721,11 +1721,13 @@ adjust_point_for_property (EMACS_INT las
      user can keep inserting another character at point or keep
      deleting characters around point.  */
   int check_composition = ! modified, check_display = 1, check_invisible = 1;
+  int check_glyphless = !modified;
   EMACS_INT orig_pt = PT;
 
   /* FIXME: cycling is probably not necessary because these properties
      can't be usefully combined anyway.  */
-  while (check_composition || check_display || check_invisible)
+  while (check_composition || check_display || check_invisible
+        || check_glyphless)
     {
       /* FIXME: check `intangible'.  */
       if (check_composition
@@ -1733,7 +1735,7 @@ adjust_point_for_property (EMACS_INT las
          && (beg = composition_adjust_point (last_pt, PT)) != PT)
        {
          SET_PT (beg);
-         check_display = check_invisible = 1;
+         check_display = check_invisible = check_glyphless = 1;
        }
       check_composition = 0;
       if (check_display
@@ -1752,7 +1754,7 @@ adjust_point_for_property (EMACS_INT las
          SET_PT (PT < last_pt
                  ? (STRINGP (val) && SCHARS (val) == 0 ? beg - 1 : beg)
                  : end);
-         check_composition = check_invisible = 1;
+         check_composition = check_invisible = check_glyphless = 1;
        }
       check_display = 0;
       if (check_invisible && PT > BEGV && PT < ZV)
@@ -1823,7 +1825,7 @@ adjust_point_for_property (EMACS_INT las
                         was already in the range: we don't get to choose
                         which end of the range we have to go to.  */
                      : (PT < last_pt ? beg : end));
-             check_composition = check_display = 1;
+             check_composition = check_display = check_glyphless = 1;
            }
 #if 0 /* This assertion isn't correct, because SET_PT may end up setting
         the point to something other than its argument, due to
@@ -1836,9 +1838,11 @@ adjust_point_for_property (EMACS_INT las
          if (!modified && !ellipsis && beg < end)
            {
              if (last_pt == beg && PT == end && end < ZV)
-               (check_composition = check_display = 1, SET_PT (end + 1));
+               (check_composition = check_display = check_glyphless = 1,
+                SET_PT (end + 1));
              else if (last_pt == end && PT == beg && beg > BEGV)
-               (check_composition = check_display = 1, SET_PT (beg - 1));
+               (check_composition = check_display = check_glyphless = 1,
+                SET_PT (beg - 1));
              else if (PT == ((PT < last_pt) ? beg : end))
                /* We've already moved as far as we can.  Trying to go
                   to the other end would mean moving backwards and thus
@@ -1851,11 +1855,30 @@ adjust_point_for_property (EMACS_INT las
                           (make_number (PT == beg ? end : beg),
                            Qinvisible, Qnil),
                           !TEXT_PROP_MEANS_INVISIBLE (val)))
-               (check_composition = check_display = 1,
+               (check_composition = check_display = check_glyphless = 1,
                 SET_PT (PT == beg ? end : beg));
            }
        }
       check_invisible = 0;
+      if (check_glyphless)
+       {
+         Lisp_Object glyphless_method;
+
+         check_glyphless = 0;
+         if (PT >= BEGV && PT < ZV)
+           {
+             glyphless_method =
+               glyphless_char_display_method (FETCH_CHAR (PT_BYTE),
+                                              XFRAME (selected_frame));
+
+             if (EQ (glyphless_method, Qzero_width))
+               {
+                 SET_PT (PT > BEGV && PT < last_pt ? PT - 1 : PT + 1);
+                 check_glyphless = 1;
+                 check_composition = check_display = check_invisible = 1;
+               }
+           }
+       }
     }
 }
 

=== modified file 'src/xdisp.c'
--- src/xdisp.c 2011-08-06 11:49:35 +0000
+++ src/xdisp.c 2011-08-06 14:00:43 +0000
@@ -770,7 +770,8 @@ Lisp_Object Qglyphless_char;
 static Lisp_Object Qglyphless_char_display;
 
 /* Method symbols for Vglyphless_char_display.  */
-static Lisp_Object Qhex_code, Qempty_box, Qthin_space, Qzero_width;
+static Lisp_Object Qhex_code, Qempty_box, Qthin_space;
+Lisp_Object Qzero_width;
 
 /* Default pixel width of `thin-space' display method.  */
 #define THIN_SPACE_WIDTH 1
@@ -6019,16 +6020,11 @@ static int (* get_next_element[NUM_IT_ME
                                 FACE_FROM_ID ((IT)->f, (IT)->face_id), \
                                 (IT)->string)))
 
-
-/* Lookup the char-table Vglyphless_char_display for character C (-1
-   if we want information for no-font case), and return the display
-   method symbol.  By side-effect, update it->what and
-   it->glyphless_method.  This function is called from
-   get_next_display_element for each character element, and from
-   x_produce_glyphs when no suitable font was found.  */
-
+/* A subroutine of lookup_glyphless_char_display.  Also called from
+   keyboard.c, to adjust point position when it is on a glyphless
+   character whose display uses the zero-width method.  */
 Lisp_Object
-lookup_glyphless_char_display (int c, struct it *it)
+glyphless_char_display_method (int c, struct frame *f)
 {
   Lisp_Object glyphless_method = Qnil;
 
@@ -6039,13 +6035,27 @@ lookup_glyphless_char_display (int c, st
        {
          glyphless_method = CHAR_TABLE_REF (Vglyphless_char_display, c);
          if (CONSP (glyphless_method))
-           glyphless_method = FRAME_WINDOW_P (it->f)
+           glyphless_method = FRAME_WINDOW_P (f)
              ? XCAR (glyphless_method)
              : XCDR (glyphless_method);
        }
       else
        glyphless_method = XCHAR_TABLE (Vglyphless_char_display)->extras[0];
     }
+  return glyphless_method;
+}
+
+/* Lookup the char-table Vglyphless_char_display for character C (-1
+   if we want information for no-font case), and return the display
+   method symbol.  By side-effect, update it->what and
+   it->glyphless_method.  This function is called from
+   get_next_display_element for each character element, and from
+   x_produce_glyphs when no suitable font was found.  */
+
+Lisp_Object
+lookup_glyphless_char_display (int c, struct it *it)
+{
+  Lisp_Object glyphless_method = glyphless_char_display_method (c, it->f);
 
  retry:
   if (NILP (glyphless_method))