The newest message is not shown.
The viewport doesn't change position as new messages are added. This makes new messages appear off-screen (lower), and since switching to a new room adds messages one by one, only the few oldest messages are on the screen, with the freshest ones off the screen below too.
Commit: e7679104 , Fedora 28, deps from source: GTK+ 3.24.1, libhandy, cairo-1.16.0.