Compact headers worked (for years) in conjunction with the ToggleHeaders add-on.
The specific behavior (on Mac Version 38.6.0):
When I forward emails I expect the header setting to appear in the forwarded email. Thus, when I expand handers to full headers (either using "View->Headers->All or with the ToggleHeaders "h" key), I expect full headers to appear in my forwarded message. Such was the former behavior. Now CompactHeaders compacts the headers (to a medium length header) and doesn't allow me to forward the full header.
For this reason I've disabled CompactHeaders until a fix is available. Thus my 5-star rating is now a 3.
This review is for a previous version of the add-on (2.1.0).