For some reason Gtk3 has a minimum size for lists of 6 items or so. That makes the whole CS frame including the filter disappear before the list when I resize via the GtkPaned splitter. It's not a problem with gen/kf5 (no idea about win/macos).
I don't know what a CS frame is so my attempt to see what new GTK3 specific issue has been discovered is stymied
CS = Character Style, the frame is the GtkFrame holding this list. The ticket was filed during GSoC when PS (paragraph style) and CS were merged into one Text Style deck. Let's resolve INVALID for now since the whole project was a flop.