I would like a simple and quick way to set and change the completion date.
I would like to use minimal clicks & don't bump between different GUI (visual context), want to stay focused on my process:
a) List View : (double-)click on the icon and that sets the CompletedDate on today, then hover over CompletedDate and 'pop-up' options: clear (currently "X" on red) and "Change", second opens the date picker, set the date
b) select date from left side (currently called "Due Date"), drag to & hold over the task, 'pop-up' show up "Start Date" - "Due Date" - "Completed Date" as options, release on "Completed Date" set the CompletedDate and set Completed
c) Task Details : always show CompletedDate input, and when setting it automatically set Completed (currently only showed when CompletedDate is already set)

I usually do this in batch, during daily status & follow-up (sometimes more seldom :-/ ).
Btw, the same time I would like to set the actual time I spent working on the task, if haven't yet.
/I assume this is already a draft user-story :-) /

Currently:
1 - (double)click on the icon front of task title to set Completed
2 - hover and click the Pen icon (Edit) -> Task Details opens
3 - click on CompletedDate's date picker
4 - click on date
5 - close Task Details - to get back to previous visual context (List) and continue with the next task on the list
... that's slow and takes away my focus at each task.

Cheers!