I check the completed box after a task is completed, but it remains (with checked box) in the active task list. What switch or routine hides a completed task?
there are two reasons why a task isn't being moved to history and removed from the list:
1. If it's a recurring task, after completion, it will update its due date to the next occurrence according to the formula used. Such tasks can be deleted by choosing "Delete task" from the gear icon menu (visible when hovering over a task name) or by selecting the task (or multiple tasks) while holding the CTRL key and then choosing "Delete" from the top menu. If you just want to move it to history, use the "Move to history" option from the gear menu.
2. If it's a sub-task, it will be moved to history once its parent task is completed (which will move the whole "tree" to history along with all sub-tasks associated with it). In such case, in order to force it to move to history, please hold down the SHIFT key while checking it off.
Shift key works. Thanks!