Fix WaveOverlayContainer always being present

osu.Game/Overlays/WaveOverlayContainer.cs

@@ -25,13 +25,20 @@ protected WaveOverlayContainer()
         protected override void PopIn()
         {
             base.PopIn();
+
             Waves.Show();
+
+            this.FadeIn();
         }
 
         protected override void PopOut()
         {
             base.PopOut();
+
             Waves.Hide();
+
+            // this is required or we will remain present even though our waves are hidden.
+            this.Delay(WaveContainer.DISAPPEAR_DURATION).FadeOut();
         }
     }
 }