Thanks. I think I may be on the way to figuring out the cause of the problem.
I managed to get my messages readable again by clicking the "-" button on the message that was stuck in the reading pane, then "+" again. Then I was able to select and read other messages again.
I reproduced the problem later by opening a certain message that got "stuck" in the reading pane again. Turns out it's Facebook messages causing it, I think. Since these are HTML messages with a lot of images embedded, I'm guessing something in the message causes a focus glitch in Evolution?
