Fix OsuButton event binding + inital-enablement colour

osu.Game.Tests/Visual/UserInterface/TestSceneOsuButton.cs

@@ -0,0 +1,46 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using NUnit.Framework;
+using osu.Framework.Graphics;
+using osu.Game.Graphics.UserInterface;
+using osuTK;
+
+namespace osu.Game.Tests.Visual.UserInterface
+{
+    public class TestSceneOsuButton : OsuTestScene
+    {
+        [Test]
+        public void TestToggleEnabled()
+        {
+            OsuButton button = null;
+
+            AddStep("add button", () => Child = button = new OsuButton
+            {
+                Anchor = Anchor.Centre,
+                Origin = Anchor.Centre,
+                Size = new Vector2(200),
+                Text = "Button"
+            });
+
+            AddToggleStep("toggle enabled", toggle =>
+            {
+                for (int i = 0; i < 6; i++)
+                    button.Action = toggle ? () => { } : (Action)null;
+            });
+        }
+
+        [Test]
+        public void TestInitiallyDisabled()
+        {
+            AddStep("add button", () => Child = new OsuButton
+            {
+                Anchor = Anchor.Centre,
+                Origin = Anchor.Centre,
+                Size = new Vector2(200),
+                Text = "Button"
+            });
+        }
+    }
+}