(column_number_displayed): New variable.

(redisplay, redisplay_window): If that var is set, do mode line.
(display_mode_line): Unset the var.
(decode_mode_spec): Set the var if %c was seen.

src/xdisp.c

@@ -200,6 +200,9 @@ int windows_or_buffers_changed;
    and it displayed a line number.  */
 int line_number_displayed;
 
+/* Nonzero if display_mode_line needs to handle %c.  */
+int column_number_displayed;
+
 /* Maximum buffer size for which to display line numbers.  */
 int line_number_display_limit;
 
@@ -583,7 +586,7 @@ redisplay ()
       frame_garbaged = 0;
     }
 
-  if (clip_changed || windows_or_buffers_changed)
+  if (clip_changed || windows_or_buffers_changed || column_number_displayed)
     update_mode_lines++;
 
   /* Detect case that we need to write a star in the mode line.  */
@@ -1356,7 +1359,7 @@ redisplay_window (window, just_this_one)
        /* If window not full width, must redo its mode line
 	  if the window to its side is being redone */
        || (!just_this_one && width < FRAME_WIDTH (f) - 1)
-       || INTEGERP (w->base_line_pos))
+       || INTEGERP (w->base_line_pos) || column_number_displayed)
       && height != XFASTINT (w->height))
     display_mode_line (w);
   if (! line_number_displayed
@@ -2717,6 +2720,7 @@ display_mode_line (w)
   register FRAME_PTR f = XFRAME (WINDOW_FRAME (w));
 
   line_number_displayed = 0;
+  column_number_displayed = 0;
 
   get_display_line (f, vpos, left);
   display_mode_element (w, vpos, left, 0, right, right,
@@ -3024,6 +3028,7 @@ decode_mode_spec (w, c, maxwidth)
       break;
 
     case 'c':
+      column_number_displayed = 1;
       sprintf (decode_mode_spec_buf, "%d", current_column ());
       return decode_mode_spec_buf;