Fix slider ball not tracking when mouse hasn't been moved since slider appeared.

osu.Game.Mode.Osu/Objects/Drawables/DrawableSlider.cs

@@ -6,6 +6,7 @@
 using osu.Game.Modes.Objects.Drawables;
 using osu.Game.Modes.Osu.Objects.Drawables.Pieces;
 using OpenTK;
+using osu.Framework.Input;
 
 namespace osu.Game.Modes.Osu.Objects.Drawables
 {
@@ -39,17 +40,14 @@ public DrawableSlider(Slider s) : base(s)
                 },
                 bouncer1 = new SliderBouncer(slider, false) { Position = slider.Curve.PositionAt(1) },
                 bouncer2 = new SliderBouncer(slider, true) { Position = slider.Position },
-                ball = new SliderBall(slider),
                 initialCircle = new DrawableHitCircle(new HitCircle
                 {
                     StartTime = s.StartTime,
                     Position = s.Position,
                     Colour = s.Colour,
                     Sample = s.Sample,
-                })
-                {
-                    Depth = -1 //override time-based depth.
-                },
+                }),
+                ball = new SliderBall(slider),
             };
 
             components.Add(body);
@@ -104,14 +102,22 @@ protected override void UpdateInitialState()
         {
             base.UpdateInitialState();
             body.Alpha = 1;
+
+            //we need to be visible to handle input events. note that we still don't get enough events (we don't get a position if the mouse hasn't moved since the slider appeared).
+            ball.Alpha = 0.01f; 
         }
 
         protected override void UpdateState(ArmedState state)
         {
             base.UpdateState(state);
 
+            ball.FadeIn();
+
             Delay(HitObject.Duration, true);
+
             body.FadeOut(160);
+            ball.FadeOut(160);
+
             FadeOut(800);
         }
     }

osu.Game.Mode.Osu/Objects/Drawables/Pieces/SliderBall.cs

@@ -98,17 +98,18 @@ public bool Tracking
             }
         }
 
+        private bool validTrackingTime => Time.Current >= slider.StartTime && Time.Current <= slider.EndTime;
+
         protected override void Update()
         {
             base.Update();
 
             CornerRadius = DrawWidth / 2;
-            Tracking = lastState != null && Contains(lastState.Mouse.NativeState.Position) && lastState.Mouse.HasMainButtonPressed;
+            Tracking = validTrackingTime && lastState != null && Contains(lastState.Mouse.NativeState.Position) && lastState.Mouse.HasMainButtonPressed;
         }
 
         public void UpdateProgress(double progress, int repeat)
         {
-            Alpha = Time.Current >= slider.StartTime && Time.Current <= slider.EndTime ? 1 : 0;
             Position = slider.Curve.PositionAt(progress);
         }
     }

osu.Game.Mode.Osu/UI/OsuPlayfield.cs

@@ -51,6 +51,7 @@ public OsuPlayfield()
 
         public override void Add(DrawableHitObject h)
         {
+            h.Depth = (float)h.HitObject.StartTime;
             DrawableHitCircle c = h as DrawableHitCircle;
             if (c != null)
             {

osu.Game/Modes/Objects/Drawables/DrawableHitObject.cs

@@ -30,7 +30,6 @@ public abstract class DrawableHitObject : Container, IStateful<ArmedState>
         public DrawableHitObject(HitObject hitObject)
         {
             HitObject = hitObject;
-            Depth = (float)hitObject.StartTime;
         }
 
         private ArmedState state;