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

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

@@ -0,0 +1,110 @@
+using System;
+using osu.Framework;
+using osu.Framework.Configuration;
+using osu.Framework.Graphics;
+using osu.Framework.Graphics.Containers;
+using osu.Framework.Input;
+using osu.Framework.Threading;
+using OpenTK;
+
+namespace osu.Game.Graphics.UserInterface.Volume
+{
+    internal class VolumeControl : OverlayContainer
+    {
+        public BindableDouble VolumeGlobal { get; set; }
+        public BindableDouble VolumeSample { get; set; }
+        public BindableDouble VolumeTrack { get; set; }
+
+        private VolumeMeter volumeMeterMaster;
+
+        public override bool Contains(Vector2 screenSpacePos) => true;
+
+        private void volumeChanged(object sender, EventArgs e)
+        {
+            Show();
+            schedulePopOut();
+        }
+
+        public VolumeControl()
+        {
+            AutoSizeAxes = Axes.Both;
+            Anchor = Anchor.BottomRight;
+            Origin = Anchor.BottomRight;
+        }
+
+        public override void Load(BaseGame game)
+        {
+            VolumeGlobal.ValueChanged += volumeChanged;
+            VolumeSample.ValueChanged += volumeChanged;
+            VolumeTrack.ValueChanged += volumeChanged;
+
+            Children = new Drawable[]
+            {
+                new FlowContainer
+                {
+                    AutoSizeAxes = Axes.Both,
+                    Anchor = Anchor.BottomRight,
+                    Origin = Anchor.BottomRight,
+                    Position = new Vector2(10, 30),
+                    Spacing = new Vector2(15,0),
+                    Children = new Drawable[]
+                    {
+                        volumeMeterMaster = new VolumeMeter("Master", VolumeGlobal),
+                        new VolumeMeter("Effects", VolumeSample),
+                        new VolumeMeter("Music", VolumeTrack)
+                    }
+                }
+            };
+
+            base.Load(game);
+        }
+
+        protected override void Dispose(bool isDisposing)
+        {
+            VolumeGlobal.ValueChanged -= volumeChanged;
+            VolumeSample.ValueChanged -= volumeChanged;
+            VolumeTrack.ValueChanged -= volumeChanged;
+            base.Dispose(isDisposing);
+        }
+
+        protected override bool OnWheelDown(InputState state)
+        {
+            if (!IsVisible)
+                return false;
+
+            volumeMeterMaster.TriggerWheelDown(state);
+            return true;
+        }
+
+        protected override bool OnWheelUp(InputState state)
+        {
+            if (!IsVisible)
+                return false;
+
+            volumeMeterMaster.TriggerWheelUp(state);
+            return true;
+        }
+
+        ScheduledDelegate popOutDelegate;
+
+        protected override void PopIn()
+        {
+            ClearTransformations();
+            FadeIn(100);
+
+            schedulePopOut();
+        }
+
+        protected override void PopOut()
+        {
+            FadeOut(100);
+        }
+
+        private void schedulePopOut()
+        {
+            popOutDelegate?.Cancel();
+            Delay(1000);
+            popOutDelegate = Schedule(Hide);
+        }
+    }
+}

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);
+        }
+    }
+}

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);
+    }
+}