add OverlayActivation enum

+ fix Toolbar being toggleable when it shouldn't be able to
+ allow opening overlays in MenuState.Initial again

osu.Game/Overlays/OverlayActivation.cs

@@ -0,0 +1,12 @@
+// Copyright (c) 2007-2018 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+namespace osu.Game.Overlays
+{
+    public enum OverlayActivation
+    {
+        Disabled,
+        //UserTriggered, // currently there is no way to discern user action
+        All
+    }
+}

osu.Game/Overlays/Toolbar/Toolbar.cs

@@ -10,6 +10,8 @@ using osu.Framework.Input;
 using osu.Game.Graphics;
 using OpenTK;
 using osu.Framework.Graphics.Shapes;
+using osu.Framework.Allocation;
+using osu.Framework.Configuration;
 
 namespace osu.Game.Overlays.Toolbar
 {
@@ -29,6 +31,11 @@ namespace osu.Game.Overlays.Toolbar
         private const float alpha_hovering = 0.8f;
         private const float alpha_normal = 0.6f;
 
+        /// <summary>
+        /// Defaults to <see cref="OverlayActivation.All"/> so that the overlay works even if BDL couldn't pass an <see cref="OsuGame"/>.
+        /// </summary>
+        private readonly Bindable<OverlayActivation> allowOverlays = new Bindable<OverlayActivation>(OverlayActivation.All);
+
         public Toolbar()
         {
             Children = new Drawable[]
@@ -76,6 +83,19 @@ namespace osu.Game.Overlays.Toolbar
             Size = new Vector2(1, HEIGHT);
         }
 
+        [BackgroundDependencyLoader(true)]
+        private void load(OsuGame osuGame)
+        {
+            if (osuGame != null)
+                allowOverlays.BindTo(osuGame.AllowOverlays);
+
+            StateChanged += visibility =>
+            {
+                if (allowOverlays == OverlayActivation.Disabled)
+                    State = Visibility.Hidden;
+            };
+        }
+
         public class ToolbarBackground : Container
         {
             private readonly Box solidBackground;