Diane,
Thank you for the advice. I was going crazy, but now I know something is wrong. Clicking on 'Thread View' causes the button to disappear to the left for a fraction of a second, but it comes right back. Occasionally when double-clicking on the 'Thread View' button, the 'View Settings' gear icon appeared very briefly before 'Thread View' button returned. However, I haven't been able to reproduce that behavior in recent tests. The reading pane can't be turned off, but that seems consistent with the description of 'Thread View' mode.
Is there any other way to turn 'Thread View' mode off?
The affected folder is very large at ~58,000 items, primarily modest sized text-based emails. I wonder if that could be an issue. I could try testing on a small test folder, but I don't remember how I turned 'Thread View' on.
-Chris