Foldback: Sync nav buttons when a new foldback bus is added

gtk2_ardour/mixer_ui.cc

@@ -613,6 +613,7 @@ Mixer_UI::add_stripables (StripableList& slist)
 					if (foldback_strip) {
 						// last strip created is shown
 						foldback_strip->set_route (route);
+						foldback_strip->prev_next_changed();
 					} else {
 						foldback_strip = new FoldbackStrip (*this, _session, route);
 						out_packer.pack_start (*foldback_strip, false, false);