Implement chatline background altering

osu.Game/Overlays/Chat/ChatLine.cs

@@ -69,6 +69,29 @@ namespace osu.Game.Overlays.Chat
 
         private Container? highlight;
 
+        private Drawable? background;
+
+        private bool alteringBackground;
+
+        public bool AlteringBackground
+        {
+            get => alteringBackground;
+            set
+            {
+                alteringBackground = value;
+
+                if (background == null)
+                    AddInternal(background = new Box
+                    {
+                        BypassAutoSizeAxes = Axes.Both,
+                        RelativeSizeAxes = Axes.Both,
+                        Colour = Color4.White,
+                    });
+
+                background.Alpha = value ? 0.04f : 0f;
+            }
+        }
+
         /// <summary>
         /// The colour used to paint the author's username.
         /// </summary>

osu.Game/Overlays/Chat/DrawableChannel.cs

@@ -104,6 +104,13 @@ namespace osu.Game.Overlays.Chat
             highlightedMessage.Value = null;
         });
 
+        private void processMessageBackgroundAltering()
+        {
+            for (int i = 0; i < ChatLineFlow.Count(); i++)
+                if (ChatLineFlow[i] is ChatLine chatline)
+                    chatline.AlteringBackground = i % 2 == 0;
+        }
+
         protected override void Dispose(bool isDisposing)
         {
             base.Dispose(isDisposing);
@@ -158,6 +165,7 @@ namespace osu.Game.Overlays.Chat
                 scroll.ScrollToEnd();
 
             processMessageHighlighting();
+            processMessageBackgroundAltering();
         });
 
         private void pendingMessageResolved(Message existing, Message updated) => Schedule(() =>