Added a comment

src/gui/widgets/timeline/timeline-layout-helper.cpp

@@ -236,6 +236,8 @@ TimelineLayoutHelper::drag_to_point(const Gdk::Point &mouse_point)
       apply_drop_to_layout_tree(drop);
       dropPoint = drop;
       
+      // Expand the branch if the user is hovering to add the track
+      // as a child
       if((drop.relation == FirstChild || drop.relation == LastChild) &&
         !target_timeline_track->get_expanded())
           target_timeline_track->expand_collapse(Track::Expand);