See original report: https://bugs.eclipse.org/bugs/show_bug.cgi?id=543086

As the discussion goes, content assist is not expected to be shown for the Expressions view as it doesn't work.

We should either prevent the pop-up (according to the discussion on the original ticket, it might be some focus bug) or fix content assist in the view.

Here is a screen recording on how to bring up the content assist window in the Expressions view:

I.e. select input text and then invoke content assist. Its not trivial to get the content assist window to show up.