However one feature that I am missing, (quite badly in fact, might need
to work out how to code plug-ins or something) is to be able to have
multiple rows of tabs in the selector widget.
I am another user that would find multi-row tabs pretty useful, but the 'Show Document List' toolbar menu works fine for me. As you say, it would be a lot of work to implement that GTK widget. But if someone built it (as a plugin, or maybe as a user preference in core Geany), I would definitely use it.
Nathan