#GNOME v50 will disable pasting via middle-mouse-button by default (as it is an X11ism):
https://gitlab.gnome.org/GNOME/gsettings-desktop-schemas/-/merge_requests/119
But one can re-enabled it via
gsettings set org.gnome.desktop.interface gtk-enable-primary-paste true
(phew, was already worried as I am using this quite often)
