Well, changing various settings (20 minutes of changing and testing), nothing solved the message display problem. But I did make a few observations that may or may not help.
1) On a message with attachments, if I swipe from right to left once, the message body disappears, the panels pager appears then fades out as the list of attachments fades in. If I swipe right to left again, the list of attachments disappears, the pager reappears, then fades out again as the list of attachments fades in again.
2) On a message with attachments, swiping left to right several times has the same effect, but on the message body rather than the attachments list.
3) On a message without attachments, the message body (not initially visible) fades in if I swipe either right to left or left to right. If I swipe again (in either direction), the text disappears then fades back in again.
4) As observed by Dan12345, if I turn off the option to fade the pager, messages with attachments do not display either text or attachments list, just the pager.
It all gives the impression that the trigger for the text fading in is the pager completely fading out and, because there is no pager on a message without attachments, it never gets that trigger to fade in.
Might it be worth trying adding an additional option, to not display the pager at all (but still be able to swipe to see the attachments list)? Or maybe have an option to force initial display of the pager even if there are no attachments?
