The BLOCK/UNBLOCK_INPUT is needed for the GTK version. This was discussed
here
http://lists.gnu.org/archive/html/emacs-pretest-bug/2004-05/msg00155.html
The solution proposed was to do as the menu code does, first build the
structures from Lisp code, then update the GUI. Only the second stage needs to
block input. Unfortunately this has not been done yet (on my todo list).
So the cause is a real bug, albeit not one unknown.
When do you think you'll be able to do make this change?
We need it before the release. (I will add it to FOR-RELEASE.)