More comments.

osu.Game/Beatmaps/Drawable/BeatmapGroup.cs

@@ -114,6 +114,7 @@ namespace osu.Game.Beatmaps.Drawable
         {
             State = BeatmapGroupState.Expanded;
 
+            //we want to make sure one of our children is selected in the case none have been selected yet.
             if (SelectedPanel == null)
                 BeatmapPanels.First().State = PanelSelectedState.Selected;
         }