add a note why preffered-time-domain is NOT editable from rc_option_editor

* if the user has explicitly made any ruler settings, they will be found
   and this will not take effect

gtk2_ardour/editor_actions.cc

@@ -1812,6 +1812,7 @@ Editor::parameter_changed (std::string p)
 		queue_redisplay_track_views ();
 	} else if (p == "default-time-domain") {
 		stretch_marker_cb.set_sensitive (_session->config.get_default_time_domain () == Temporal::BeatTime);
+		restore_ruler_visibility();  /* NOTE: if user has explicitly set rulers then this will have no effect */
 	}
 }
 

gtk2_ardour/rc_option_editor.cc

@@ -2388,6 +2388,12 @@ RCOptionEditor::RCOptionEditor ()
 					    _("When opening an existing session, if the most recent audio engine is available and can open the session's sample rate, the audio engine dialog may be skipped."));
 	add_option (_("General"), bo );
 
+#if 0
+	/* "preferred-time-domain"
+	   unlike other preferences, this variable would not actually 'change' anything; the session's domain is stored in session-config
+	   so we don't show the rc-config value here.  instead, it is invisibly stored in the rc-config, with the user's most recent selection */
+#endif
+
 	add_option (_("General"), new OptionEditorHeading (S_("Options|Editor Undo")));
 
 	add_option (_("General"), new UndoOptions (_rc_config));

gtk2_ardour/session_dialog.cc

@@ -676,11 +676,7 @@ SessionDialog::setup_new_session_page ()
 
 	timebase_chooser.append (_("Audio Time"));
 	timebase_chooser.append (_("Beat Time"));
-#ifdef MIXBUS
-	timebase_chooser.set_active (1);
-#else
-	timebase_chooser.set_active (0);
-#endif
+	timebase_chooser.set_active (Config->get_preferred_time_domain() == Temporal::BeatTime ? 1 : 0);
 
 	//Template & Template Description area
 	HBox* template_hbox = manage (new HBox);

libs/ardour/ardour/rc_configuration_vars.h

@@ -30,6 +30,8 @@
     the value of the variable.
 *****************************************************/
 
+CONFIG_VARIABLE (Temporal::TimeDomain, preferred_time_domain, "preferred_time_domain", Temporal::BeatTime)
+
 /* IO connection */
 
 CONFIG_VARIABLE (bool, auto_connect_standard_busses, "auto-connect-standard-busses", true)

libs/ardour/session_state.cc

@@ -4588,6 +4588,7 @@ Session::config_changed (std::string p, bool ours)
 		Temporal::TimeDomain td = config.get_default_time_domain ();
 		std::cerr << "Setting time domain\n";
 		set_time_domain (td);
+		Config->set_preferred_time_domain(td);  /* sync the global default time domain to this newly chosen one */
 	}
 
 	set_dirty ();