Hack around this bug:

http://lists.gnu.org/archive/html/bug-bash/2004-10/msg00419.html

not suitable for upstream; need a proper fix

http://bugs.gentoo.org/108936
http://bugs.gentoo.org/69407

--- lib/readline/display.c
+++ lib/readline/display.c
@@ -941,6 +941,7 @@
 	  /* nleft == number of characters in the line buffer between the
 	     start of the line and the cursor position. */
 	  nleft = c_pos - pos;
+#if 0
 
 	  /* Since _rl_backspace() doesn't know about invisible characters in the
 	     prompt, and there's no good way to tell it, we compensate for
@@ -954,6 +955,8 @@
 		_rl_last_c_pos = nleft;
 	    }
 
+#endif
+
 	  if (MB_CUR_MAX > 1 && rl_byte_oriented == 0)
 	    _rl_move_cursor_relative (nleft, &invisible_line[pos]);
 	  else if (nleft != _rl_last_c_pos)