Mark single messages read only

app/src/main/java/eu/faircode/email/ActivityView.java

@@ -746,8 +746,10 @@ public class ActivityView extends ActivityBilling implements FragmentManager.OnB
                     if (!message.content)
                         EntityOperation.queue(db, message, EntityOperation.BODY);
 
-                    db.message().setMessageUiSeen(message.id, true);
-                    EntityOperation.queue(db, message, EntityOperation.SEEN, true);
+                    if (!message.threaded) {
+                        db.message().setMessageUiSeen(message.id, true);
+                        EntityOperation.queue(db, message, EntityOperation.SEEN, true);
+                    }
 
                     db.setTransactionSuccessful();
                 } finally {