add a descriptive tooltip to the Timebase selector (needs translations)

gtk2_ardour/session_dialog.cc

@@ -678,6 +678,14 @@ SessionDialog::setup_new_session_page ()
 	timebase_chooser.append (_("Beat Time"));
 	timebase_chooser.set_active (Config->get_preferred_time_domain() == Temporal::BeatTime ? 1 : 0);
 
+	set_tooltip (timebase_chooser, _(
+	"The timebase controls how some items on the timeline respond to tempo map editing.\n\n"
+	"If you choose Beat Time, some items (like markers) will move when you change tempo.\n\n"
+	"If you choose Audio Time, these items will not move when you change tempo.\n\n"
+	"The timebase also affects which ruler lanes will be initially shown.\n\n"
+	"You can change the session's timebase anytime in Session->Properties." 
+		));
+
 	//Template & Template Description area
 	HBox* template_hbox = manage (new HBox);
 

libs/ardour/ardour/session_configuration_vars.h

@@ -91,4 +91,8 @@ CONFIG_VARIABLE (bool, show_monitor_on_meterbridge, "show-monitor-on-meterbridge
 CONFIG_VARIABLE (bool, show_name_on_meterbridge, "show-name-on-meterbridge", true)
 CONFIG_VARIABLE (bool, show_fader_on_meterbridge, "show-fader-on-meterbridge", false)
 CONFIG_VARIABLE (uint32_t, meterbridge_label_height,  "meterbridge-label-height", 0)
-CONFIG_VARIABLE (Temporal::TimeDomain, default_time_domain, "default-time-domain", Temporal::AudioTime)
+
+/* If the user changes the session default_time_domain, we also stash that in rc_config as a global preference,
+     where it is used to initialize the session timebase menu during new session creation
+*/
+CONFIG_VARIABLE (Temporal::TimeDomain, default_time_domain, "default-time-domain", Temporal::BeatTime)