Reduce initial channel load overhead by only loading history on active channel

osu.Game/Online/Chat/ChannelManager.cs

@@ -93,6 +93,12 @@ private void currentChannelChanged(ValueChangedEvent<Channel> e)
         {
             if (!(e.NewValue is ChannelSelectorTabItem.ChannelSelectorTabChannel))
                 JoinChannel(e.NewValue);
+
+            if (e.NewValue?.MessagesLoaded == false)
+            {
+                // let's fetch a small number of messages to bring us up-to-date with the backlog.
+                fetchInitalMessages(e.NewValue);
+            }
         }
 
         /// <summary>
@@ -375,12 +381,6 @@ public Channel JoinChannel(Channel channel, bool alreadyJoined = false)
             if (CurrentChannel.Value == null)
                 CurrentChannel.Value = channel;
 
-            if (!channel.MessagesLoaded)
-            {
-                // let's fetch a small number of messages to bring us up-to-date with the backlog.
-                fetchInitalMessages(channel);
-            }
-
             return channel;
         }
 

osu.Game/Overlays/Chat/DrawableChannel.cs

@@ -105,6 +105,14 @@ protected override void Dispose(bool isDisposing)
 
         private void newMessagesArrived(IEnumerable<Message> newMessages)
         {
+            if (newMessages.Min(m => m.Id) < chatLines.Max(c => c.Message.Id))
+            {
+                // there is a case (on initial population) that we may receive past messages and need to reorder.
+                // easiest way is to just combine messages and recreate drawables (less worrying about day separators etc.)
+                newMessages = newMessages.Concat(chatLines.Select(c => c.Message)).OrderBy(m => m.Id).ToList();
+                ChatLineFlow.Clear();
+            }
+
             bool shouldScrollToEnd = scroll.IsScrolledToEnd(10) || !chatLines.Any() || newMessages.Any(m => m is LocalMessage);
 
             // Add up to last Channel.MAX_HISTORY messages