improved performance and battery usage on older devices

lib/api/marianumcloud/talk/chat/get_chat_cache.dart

@@ -17,7 +17,7 @@ class GetChatCache extends SimpleCache<GetChatResponse> {
              lookIntoFuture: GetChatParamsSwitch.off,
              setReadMarker: GetChatParamsSwitch.on,
              // Small initial page; also the per-chat offline snapshot written to
-             // localstore. Older messages are paged in on scroll-up via
+             // the request cache. Older messages are paged in on scroll-up via
              // GetChatHistory. Keep in sync with ChatBloc's _kInitialPageSize.
              limit: 50,
            ),

lib/api/marianumcloud/talk/chat/get_chat_history.dart

@@ -1,3 +1,4 @@
+import 'package:flutter/foundation.dart';
 import 'package:http/http.dart' as http;
 
 import '../../../errors/server_exception.dart';
@@ -45,8 +46,10 @@ class GetChatHistory {
     final status = response.statusCode;
     if (status == 304) return null;
     if (status >= 200 && status < 300) {
-      return GetChatResponse.fromJson(NextcloudOcs.decode(response.body))
-        ..headers = response.headers;
+      // A page holds up to 200 messages and lands while the user is scrolling;
+      // decoding it on the UI isolate stalls the fling.
+      final parsed = await compute(_parseChatResponse, response.body);
+      return parsed..headers = response.headers;
     }
     throw ServerException(
       statusCode: status,
@@ -54,3 +57,6 @@ class GetChatHistory {
     );
   }
 }
+
+GetChatResponse _parseChatResponse(String body) =>
+    GetChatResponse.fromJson(NextcloudOcs.decode(body));