When you check off a task, it will remain visible until you change tabs, re-sort the list or refresh the page. This is on purpose because it allows you to uncheck a task if you checked it by mistake. If the task vanished instantly, it would not be possible to correct mistakes like this.
I actually don't change tabs so much. I do not want to change tab or re-sort or refresh every time I check a task as complete (which is quite frequent) because I don't need to do them. Could you kindly add an optional option in the Account settings for those who want the completed tasks to auto-hide without having to change tabs, re-sort or refresh? It would be very helpful and keep our tasks list clean without having to refresh.
That is why it can be a option in the settings. Those who want the completed tasks to disappear immediately to keep it clean without having to change tabs can set it on. The default can be as it is presently.