Added ScrollBarVisibility.Disabled.

src/Avalonia.Controls/Primitives/ScrollBar.cs

@@ -99,6 +99,7 @@ namespace Avalonia.Controls.Primitives
                 case ScrollBarVisibility.Visible:
                     return true;
 
+                case ScrollBarVisibility.Disabled:
                 case ScrollBarVisibility.Hidden:
                     return false;
 

src/Avalonia.Controls/Primitives/ScrollBarVisibility.cs

@@ -5,8 +5,9 @@ namespace Avalonia.Controls.Primitives
 {
     public enum ScrollBarVisibility
     {
+        Disabled,
         Auto,
-        Visible,
         Hidden,
+        Visible,
     }
 }