Update UI controls to understand DisabledChanged.

osu.Game/Graphics/UserInterface/OsuCheckbox.cs

@@ -25,8 +25,6 @@ namespace osu.Game.Graphics.UserInterface
             {
                 bindable = value;
                 Current.BindTo(bindable);
-                if (value?.Disabled ?? true)
-                    Alpha = 0.3f;
             }
         }
 
@@ -84,6 +82,11 @@ namespace osu.Game.Graphics.UserInterface
                 else
                     sampleUnchecked?.Play();
             };
+
+            Current.DisabledChanged += disabled =>
+            {
+                Alpha = disabled ? 0.3f : 1;
+            };
         }
 
         protected override bool OnHover(InputState state)

osu.Game/Graphics/UserInterface/OsuSliderBar.cs

@@ -74,6 +74,11 @@ namespace osu.Game.Graphics.UserInterface
                     Expanded = true,
                 }
             };
+
+            Current.DisabledChanged += disabled =>
+            {
+                Alpha = disabled ? 0.3f : 1;
+            };
         }
 
         [BackgroundDependencyLoader]

osu.Game/Graphics/UserInterface/OsuTextBox.cs

@@ -33,6 +33,11 @@ namespace osu.Game.Graphics.UserInterface
             Height = 40;
             TextContainer.Height = 0.5f;
             CornerRadius = 5;
+
+            Current.DisabledChanged += disabled =>
+            {
+                Alpha = disabled ? 0.3f : 1;
+            };
         }
 
         [BackgroundDependencyLoader]

osu.Game/Overlays/Options/OptionDropdown.cs

@@ -34,8 +34,6 @@ namespace osu.Game.Overlays.Options
             {
                 bindable = value;
                 dropdown.Current.BindTo(bindable);
-                if (value?.Disabled ?? true)
-                    Alpha = 0.3f;
             }
         }
 
@@ -75,6 +73,11 @@ namespace osu.Game.Overlays.Options
                     Items = Items,
                 }
             };
+
+            dropdown.Current.DisabledChanged += disabled =>
+            {
+                Alpha = disabled ? 0.3f : 1;
+            };
         }
     }
 }

osu.Game/Overlays/Options/OptionSlider.cs

@@ -35,8 +35,6 @@ namespace osu.Game.Overlays.Options
             {
                 bindable = value;
                 slider.Current.BindTo(bindable);
-                if (value?.Disabled ?? true)
-                    Alpha = 0.3f;
             }
         }
 

osu.Game/Overlays/Options/OptionTextBox.cs

@@ -16,9 +16,7 @@ namespace osu.Game.Overlays.Options
             {
                 bindable = value;
                 Current.BindTo(bindable);
-                if (value?.Disabled ?? true)
-                    Alpha = 0.3f;
             }
         }
     }
-}
+}