Revert remove renderMessages

I realized the use of `renderMessages` prevents stuttering when resuming
scroll.

lib/screens/channel/chat/chat.dart

@@ -42,9 +42,9 @@ class Chat extends StatelessWidget {
                               padding: EdgeInsets.zero,
                               addAutomaticKeepAlives: false,
                               controller: chatStore.scrollController,
-                              itemCount: chatStore.messages.length,
+                              itemCount: chatStore.renderMessages.length,
                               itemBuilder: (context, index) => ChatMessage(
-                                ircMessage: chatStore.messages.reversed.toList()[index],
+                                ircMessage: chatStore.renderMessages.reversed.toList()[index],
                                 chatStore: chatStore,
                               ),
                             );

lib/screens/channel/chat/stores/chat_store.dart

@@ -22,6 +22,9 @@ abstract class ChatStoreBase with Store {
   /// The total maximum amount of messages in chat.
   static const _messageLimit = 5000;
 
+  /// The maximum ammount of messages to render when autoscroll is enabled.
+  static const _renderMessageLimit = 100;
+
   /// The amount of messages to free (remove) when the [_messageLimit] is reached.
   final _messagesToRemove = (_messageLimit * 0.2).toInt();
 
@@ -91,6 +94,19 @@ abstract class ChatStoreBase with Store {
   @readonly
   var _messages = ObservableList<IRCMessage>();
 
+  /// The list of chat messages that should be rendered. Used to prevent jank when resuming scroll.
+  @computed
+  List<IRCMessage> get renderMessages {
+    // If autoscroll is disabled, render ALL messages in chat.
+    // The second condition is to prevent an out of index error with sublist.
+    if (!_autoScroll || _messages.length < _renderMessageLimit) return _messages;
+
+    // When autoscroll is enabled, only show the first [_renderMessageLimit] messages.
+    // This will improve performance by only rendering a limited amount of messages
+    // instead of the entire history at all times.
+    return _messages.sublist(_messages.length - _renderMessageLimit);
+  }
+
   /// If the chat should automatically scroll/jump to the latest message.
   @readonly
   var _autoScroll = true;

lib/screens/channel/chat/stores/chat_store.g.dart

@@ -9,6 +9,14 @@ part of 'chat_store.dart';
 // ignore_for_file: non_constant_identifier_names, unnecessary_brace_in_string_interps, unnecessary_lambdas, prefer_expression_function_bodies, lines_longer_than_80_chars, avoid_as, avoid_annotating_with_dynamic, no_leading_underscores_for_local_identifiers
 
 mixin _$ChatStore on ChatStoreBase, Store {
+  Computed<List<IRCMessage>>? _$renderMessagesComputed;
+
+  @override
+  List<IRCMessage> get renderMessages => (_$renderMessagesComputed ??=
+          Computed<List<IRCMessage>>(() => super.renderMessages,
+              name: 'ChatStoreBase.renderMessages'))
+      .value;
+
   late final _$_notificationAtom =
       Atom(name: 'ChatStoreBase._notification', context: context);
 
@@ -244,7 +252,8 @@ mixin _$ChatStore on ChatStoreBase, Store {
   @override
   String toString() {
     return '''
-expandChat: ${expandChat}
+expandChat: ${expandChat},
+renderMessages: ${renderMessages}
     ''';
   }
 }