You can mark a task as completed from anywhere!
In every SAU view there is a circle to the left of the task name, by clicking on which the task will be marked as completed.
You can even complete the task from its card. To do that:
  1. Open the projects panel and select a project
  2. Find the task you want to change and click on it. The task card will open
  3. Click on the circle to the left of the task's name. A green check mark will appear