Let's separate this bug from main thread https://bugzilla.gnome.org/show_bug.cgi?id=765516, I find it quite hard to follow the current status of each bug when they are all mixed.

> Thanks for the stack. I'm now on 3.22 and I still aren't getting those warnings however, and I don't know how I can solve it without re-introducing "How does the code know the size to allocate?" warnings that were introduced in GTK 3.20.

> I think I know what the 1px message body bug is however - do these message bodies suddenly appear if you resize the window?
Yeah, totally! Nice intuition :). Both lowering size and enlarging main window do the trick.