Added functioning tests.

osu.Game/Graphics/UserInterface/HistoryTextBox.cs

@@ -0,0 +1,73 @@
+// 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.Collections.Generic;
+using osu.Framework.Input.Events;
+using osuTK.Input;
+
+namespace osu.Game.Graphics.UserInterface
+{
+    public class HistoryTextBox : FocusedTextBox
+    {
+        private readonly List<string> messageHistory = new List<string>();
+
+        public IReadOnlyList<string> MessageHistory => messageHistory;
+
+        private int historyIndex = -1;
+
+        private string originalMessage = string.Empty;
+
+        public HistoryTextBox()
+        {
+            Current.ValueChanged += text =>
+            {
+                if (string.IsNullOrEmpty(text.NewValue))
+                    historyIndex = -1;
+            };
+        }
+
+        protected override bool OnKeyDown(KeyDownEvent e)
+        {
+            switch (e.Key)
+            {
+                case Key.Up:
+                    if (historyIndex == -1)
+                        originalMessage = Text;
+
+                    if (historyIndex == messageHistory.Count - 1)
+                        return true;
+
+                    Text = messageHistory[++historyIndex];
+
+                    return true;
+
+                case Key.Down:
+                    if (historyIndex == -1)
+                        return true;
+
+                    if (historyIndex == 0)
+                    {
+                        historyIndex = -1;
+                        Text = originalMessage;
+                        return true;
+                    }
+
+                    Text = messageHistory[--historyIndex];
+
+                    return true;
+            }
+
+            return base.OnKeyDown(e);
+        }
+
+        protected override void Commit()
+        {
+            if (!string.IsNullOrEmpty(Text))
+                messageHistory.Insert(0, Text);
+
+            historyIndex = -1;
+
+            base.Commit();
+        }
+    }
+}