Displaying current trail only

This is a question that I am sure has been answered elsewhere. I have looked nad looked but can't find a straight-forward example to follow.

I have a custom menu similar to:

Parent 1
-Child 1.1
-Child 1.2
--Child 1.2.1
Parent 2
-Child 2.1
-Child 2.2
Parent 3
-Child 3.1
-Child 3.2

My need is to have a block that, if you are anywhere in the Parent 1 trail, will show...
Parent 1
-Child 1.1
-Child 1.2
--Child 1.2.1

Anywhere in the Parent 2 trail...
Parent 2
-Child 2.1
-Child 2.2

Etc., without hard coding separate menus for each parent trail, because new parents will be added from time to time. It seems like I should be able to choose the configuration included in my screenshot, but no menu is displayed on any page with these settings.