Move border to more appropriate namespace

osu.Game/Rulesets/UI/PlayfieldBorder.cs

@@ -0,0 +1,37 @@
+// 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 osu.Framework.Graphics;
+using osu.Framework.Graphics.Containers;
+using osu.Framework.Graphics.Shapes;
+using osuTK.Graphics;
+
+namespace osu.Game.Rulesets.UI
+{
+    /// <summary>
+    /// Provides a border around the playfield.
+    /// </summary>
+    public class PlayfieldBorder : VisibilityContainer
+    {
+        private const int fade_duration = 200;
+
+        public PlayfieldBorder()
+        {
+            RelativeSizeAxes = Axes.Both;
+
+            Masking = true;
+            BorderColour = Color4.White;
+            BorderThickness = 2;
+
+            InternalChild = new Box
+            {
+                RelativeSizeAxes = Axes.Both,
+                Alpha = 0,
+                AlwaysPresent = true
+            };
+        }
+
+        protected override void PopIn() => this.FadeIn(fade_duration, Easing.OutQuint);
+        protected override void PopOut() => this.FadeOut(fade_duration, Easing.OutQuint);
+    }
+}