I tried this feature on the Roundup instance of bugs.python.org and it
didn't work out of the box.  The problem seemed to be caused by some
conflict between the words "keyword" and "keywords".
In the search page the <select> uses "keywords", but AFAIU the script
expects "keyword" in order to work, both in the "(expr)" link of the
popup and in the js code that tries to access the form using
getElementById('keyword').
I fixed it by replacing the occurrences of 'keyword' to 'keywords' where
necessary [0].  Note that in issue.search.html I just added back the 's'
that I had removed in the previous commit.  That made the popup appear
correctly, but then the search wasn't working anymore because Roundup
was expecting "keywords" (with the 's') on the other side.
My solution is probably not the most elegant (I basically hardcoded
'keyword' and 'keywords' in the right places), but it fixed the problem
for me.

Another issue I noticed is that all the keywords (included retired ones)
are listed in the popup.

[0]: http://svn.python.org/view?view=revision&revision=88877