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

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

@@ -0,0 +1,90 @@
+using osu.Framework;
+using osu.Framework.Configuration;
+using osu.Framework.Graphics;
+using osu.Framework.Graphics.Containers;
+using osu.Framework.Graphics.Sprites;
+using osu.Framework.Graphics.Transformations;
+using osu.Framework.Input;
+using OpenTK;
+using OpenTK.Graphics;
+
+namespace osu.Game.Graphics.UserInterface.Volume
+{
+    internal class VolumeMeter : Container
+    {
+        private Box meterFill;
+        private BindableDouble volume;
+
+        public VolumeMeter(string meterName, BindableDouble volume)
+        {
+            this.volume = volume;
+            Size = new Vector2(40, 180);
+            Children = new Drawable[]
+            {
+                new Box
+                {
+                    Colour = Color4.Black,
+                    RelativeSizeAxes = Axes.Both
+                },
+                new Container
+                {
+                    RelativeSizeAxes = Axes.Both,
+                    Size = new Vector2(0.5f, 0.9f),
+                    Anchor = Anchor.Centre,
+                    Origin = Anchor.Centre,
+                    Children = new Drawable[]
+                    {
+                        new Box
+                        {
+                            Colour = Color4.DarkGray,
+                            RelativeSizeAxes = Axes.Both
+                        },
+                        meterFill = new Box
+                        {
+                            Colour = Color4.White,
+                            RelativeSizeAxes = Axes.Both,
+                            Origin = Anchor.BottomCentre,
+                            Anchor = Anchor.BottomCentre
+                        }
+                    }
+                },
+                new SpriteText
+                {
+                    Text = meterName,
+                    Anchor = Anchor.BottomCentre,
+                    Origin = Anchor.TopCentre
+                }
+            };
+        }
+
+        public override void Load(BaseGame game)
+        {
+            base.Load(game);
+            updateFill();
+        }
+
+        public double Volume
+        {
+            get { return volume.Value; }
+            private set
+            {
+                volume.Value = value;
+                updateFill();
+            }
+        }
+
+        protected override bool OnWheelUp(InputState state)
+        {
+            Volume += 0.05f;
+            return true;
+        }
+
+        protected override bool OnWheelDown(InputState state)
+        {
+            Volume -= 0.05f;
+            return true;
+        }
+
+        private void updateFill() => meterFill.ScaleTo(new Vector2(1, (float)Volume), 300, EasingTypes.OutQuint);
+    }
+}