This is a known regression of the Gtk3 port. What have we learned so far: * drag and drop is broken for internal plugins from the "move" context menu (moving in the prefs dialog works, moving external plugins also works) * oddly, DND works for internal plugins too when /force-all-external is set to true * even with external plugins, DND doesn't fully work. the possible targets are only next to internal plugins, so you cannot e.g. drop an external plugin between two other external plugins * branch that could help with debugging: https://git.xfce.org/users/andrzejr/xfce4-panel/log/?h=drag_and_drop_experiments * mclasen mentioned that investigating https://developer.gnome.org/gdk3/stable/gdk3-Drag-and-Drop.html#gdk-drag-context-manage-dnd may help understanding what's going on (a lot of functionality was moved there)
Peter de Ridder referenced this bugreport in commit 5a54105b14e46bdace52917002267e97ebb21166 Fix drag and drop for internal plugins (Bug #13642) https://git.xfce.org/xfce/xfce4-panel/commit?id=5a54105b14e46bdace52917002267e97ebb21166
Yay, thanks Peter!