Thanks for the quick response!
I should clarify that most of the keys don't work. The arrow keys quit working. And things like Alt+F to try and get the "File..." menu doesn't work. It's as if the entire keyboard is frozen. The only thing that works is the mouse and Alt+Tab. I have to use the mouse to click away from the HTML file and then the keyboard is working again.
If the problem is the IE ActiveX container, is there an alternative viewer? I would prefer not to switch the HTML handler to be a plain ASCII text viewer because (obviously) the HTML files would be much harder to preview.
[edit: ok, I played around with Opus some more and it does looks like IE is stealing keyboard focus. I didn't realize this until I found an HTML file that was longer than screen length --- I then noticed that the up&down arrow keys were scrolling the HTML viewer window. Is there a keyboard combination that will set focus back to Opus Lister?]
On the other hand, viewing Acrobat PDF files does not disable the keyboard at all.
Also, it seems like more people would stumble across this problem so I'm surprised it's not mentioned in any FAQ.