I can't edit Panel variants I think this may have happened when upgrading to Drupal 8.5.x, but I can no longer edit a panel variant. 

I can add a new variant and seem to be offered the typical layout, content, etc., but nothing happens when I click a variant, and I don't have the typical menu selections (see screen shot).

I am logged in as User 1 with all permissions checked. There are no errors in the error log. I have the latest dev version of all the relevant modules (Panels, Ctools, etc.).

Any guidance would be greatly appreciated.
I had the same issue and I managed to solve it by finding the following issue/patch: https://www.drupal.org/project/page_manager/issues/2921404#comment-12526453