Impossible to re-dock the preview pane once undocked

[toobaz]

I did the mistake of clicking the "undock" button just above the preview window.

Now it floats as an independent window, and I couldn't find any way to bring it back to the default. I tried dragging, checking all menus, double clicking the title bar... nothing worked. I'm using GNOME 3 under Wayland, in case it matters.

I even tried to delete the ktikz configuration file (tried both ~/.config/ktikzrc and .config/Florian_Hackenberger/ktikz.conf) and the window remained undocked!

The exact same problem arises with qtikz (not sure what's the difference between the two, but Debian has both, independently packaged).