Highlight the groups a contact is in? There is a keyboard shortcut for "Contacts.app" which does not appear to be documented anywhere (correct me if I'm wrong) which allows you to highlight the groups that the selected contact is part of, assuming the Groups pane is visible.

You just hold down the "Option" key.

However, there is another mode that I have only ever activated by accident, where these highlights stay on.  You can click or arrow-key from one contact to another, and the highlights will change accordingly without holding down the "Option" key.

This is a really useful way to view Contacts.  Unfortunately, all my efforts to either (a) find the documentation for this, or (b) activate this mode on purpose have failed.

I am using El Capitan, but I recall seeing the same behavior all the way back in Snow Leopard, and I couldn't figure out how to make the highlighting stay on then, either.

How can I make this highlighting mode stay on, to quickly and at-a-glance show which groups a particular contact belongs to?
I think you found a bug.
Within All Contacts,
if you select one contact entry,
hold down ⌥⇧, maintain them, and select a second contact entry
then the highlighted group will stay on.

From there, if you use the keyboard arrows, this highlighting will remain on, but won't be correct.

My analysis is that in this sequence of events the ⌥ key up
isn't managed correctly.