This is still happening in 6.0.6.8021.

As before, it happened the first time I tried Ctrl+Tab.

I've attempted to reproduce this with a new simple project, but it doesn't happen. Yet it always happens in the real project that I'm working on!

I think the easiest way to demonstrate the problem would be if you download and build our project from our Subversion repository:
http://svn.openehr.org/ref_impl_eiffel/TRUNK
The project is "apps\adl_workbench\app\adl_workbench.ecf" (the "adl_workbench" target). Once you've downloaded the source, simply open adl_workbench in EiffelStudio and compile it. Then open a couple of classes. Classes like ANY and STRING don't seem to exhibit the problem, but DADL2_VALIDATOR and YY_PARSER_SKELETON hang every time!