Dragging rows in a GtkTreeView in gtk4

First reported in issue #3642 (closed).

I've found several problems with drag-and-drop in gtk4. My system uses the X11 backend. I found these problems in demos and example programs in gtkmm and gtkmm-documentation. They also occur in similar programs in gtk.

In gtk3, tests/testtreeview, rows can be reordered by dragging and dropping them. That's not possible in gtk4.