Thanks for the post. We appreciate your interest in participating in the forums! These forums are for questions about developing software and accessories for Apple platforms. Your question seems related to a consumer feature but I guess being on a beta the Apple Support Community sent you here.
I have tried in Golden Gate and open Mail in Full Screen mode. I do not see the behavior you are describing at all. I would like you to remind you that Golden Gate is in beta so you could provide Feedback using the Feedback Assistant if you see something that is not working as intended.
Once you open the bug report, please post the FB number here for my reference.
If you have any questions about filing a bug report, take a look at
Bug Reporting: How and Why?
Albert WWDR