[Feature request] Include sub-tasks in Completed list

Start with the Completed view and Calendar view, then trace how completed top-level tasks and their sub-tasks are selected and rendered. Compare that behavior with completed sub-tasks under incomplete parents, and clarify whether the proposed display should be optional. Done means the Completed view includes those sub-tasks with their incomplete parent tree greyed out while preserving the existing completed-task display.

Is your feature request related to a problem? Please describe.

Nextcloud's UI around completed tasks is confusing:

• The left bar has a menu entry, Completed with a number to the right of it, e.g. 42.
• That number is a count of completed (and/or cancelled?) tasks, irrespective of whether they are top-level tasks or sub-tasks.
• However, if the user clicks that menu entry, they are only shown:
  • completed top-level tasks, and
  • sub-tasks belonging to those top-level tasks.
• No other sub-tasks are shown in the Completed view.

Suppose the user has complete 3 top-level tasks, and 39 sub-tasks - but only with 4 of those sub-tasks belong to the completed top-level tasks. In this case, when the user visits the Completed view, they would be shown just 7 tasks (3 top-level + 4 sub-tasks of those top-level tasks): a completely different number than the 42 they were expecting.

Describe the solution you'd like

In addition to showing, as at present, completed top-level tasks and their sub-tasks, Calender view should also show completed sub-tasks belonging to incomplete top-level tasks, as follows:

• The completed sub-task should be shown at the bottom of a task tree extending to its incomplete top-level parent task. All incomplete tasks in the tree should be greyed-out, so that the visual focus of the UI is on the completed tasks.

Describe alternatives you've considered

Alternatively, completed sub-tasks of incomplete parent tasks could be shown beneath text saying "Incomplete parent task(s) not shown", but this would probably be less helpful for the user.

Additional context

Maybe this functionality should be made optional, in case there are users who prefer the existing (confusing!) UI?