Thanks for the screencast; that helps!
Your narration sounds like you’re expecting Tab-c to add a tag, but in fact that shortcut is actually Tab-t.
In your screencast at the end, holding down tab and then tapping “c” while tab is held down actually expanded the Comment textarea at the bottom of the task pane, as it is supposed to because Tab-c is for Comment.
So that manner of using tab seems to be working; try it with Tab-t.
Hope that helps,
Larry