In the feed section, you can modify a work item's status according to its context. i.e, When a task is updated, you can complete it from the feed itself; when a task is already completed, you can reopen it from the feed itself. That's how it works. The following screenshot is an example to help you understand this case.

Helen Collins completes a task at 2:44pm and the  menu says Click to open.

Whereas, she updates the completion percentage for another task at 2:51pm and its  menu says Mark as complete.