Comment # 3 on bug 1146018 from Alec Bloss

Are you running a Wayland session or using X11?

I'm going to note that some GTK3 applications have problems with copy-and-paste
when copying from GTK3 app and pasting to a KDE/Qt app, but it usually works
the other way around. This specific example doesn't exist on X11, only on
Wayland using XWayland and GTK.