Fix volume controls handling mouse wheel at a higher level than anything else game-wide.

osu.Game/Graphics/UserInterface/Volume/VolumeControlReceptor.cs

@@ -0,0 +1,42 @@
+//Copyright (c) 2007-2016 ppy Pty Ltd <contact@ppy.sh>.
+//Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+using System;
+using osu.Framework.Graphics;
+using osu.Framework.Graphics.Containers;
+using osu.Framework.Input;
+using OpenTK.Input;
+using OpenTK;
+
+namespace osu.Game.Graphics.UserInterface.Volume
+{
+    class VolumeControlReceptor : Container
+    {
+        public Action ActivateRequested;
+
+        protected override bool OnWheelDown(InputState state)
+        {
+            ActivateRequested?.Invoke();
+            return true;
+        }
+
+        protected override bool OnWheelUp(InputState state)
+        {
+            ActivateRequested?.Invoke();
+            return true;
+        }
+
+        protected override bool OnKeyDown(InputState state, KeyDownEventArgs args)
+        {
+            switch (args.Key)
+            {
+                case Key.Up:
+                case Key.Down:
+                    ActivateRequested?.Invoke();
+                    return true;
+            }
+
+            return base.OnKeyDown(state, args);
+        }
+    }
+}