tempo mapping: handle the special case of trying to ramp the 'first' marker

* the process of tempo mapping requires/encourages you to click on the
   last-known good marker, before continuing to make tempo tweaks

* this conflicts with the idea that clicking in a ruler should start a
    playhead-drag.  TBD.   but tempo-mapping is a special and rare operation.

gtk2_ardour/editor_drag.cc

@@ -3626,15 +3626,6 @@ MappingTwistDrag::motion (GdkEvent* event, bool first_move)
 void
 MappingTwistDrag::finished (GdkEvent* event, bool movement_occurred)
 {
-	if (!movement_occurred) {
-		/* click, no drag */
-
-		_editor->abort_tempo_mapping ();
-		_editor->abort_reversible_command ();
-		_editor->session ()->request_locate (grab_sample (), false, _was_rolling ? MustRoll : RollIfAppropriate);
-		return;
-	}
-
 	if (!_drag_valid) {
 		_editor->abort_tempo_mapping ();
 		_editor->abort_reversible_command ();

gtk2_ardour/editor_mouse.cc

@@ -3060,5 +3060,13 @@ Editor::choose_mapping_drag (ArdourCanvas::Item* item, GdkEvent* event)
 		std::cerr << "TWIST\n";
 		begin_reversible_command (_("tempo mapping: mid-twist"));
 		_drags->set (new MappingTwistDrag (this, item, map, *before, *focus, *after, *before_state, ramped), event);
+	} else if (ramped && focus && after) {
+		/* special case 4: user is manipulating a beat line after the INITIAL tempo marker, so there is no prior marker*/
+		std::cerr << "TWIST ON START\n";
+		begin_reversible_command (_("tempo mapping: mid-twist"));
+		before = focus; /* this is unused in MappingTwistDrag, when ramped is true, but let's not pass in garbage */
+		_drags->set (new MappingTwistDrag (this, item, map, *before, *focus, *after, *before_state, ramped), event);
+	} else {
+		abort_tempo_mapping ();  /* NOTREACHED */
 	}
 }