Scolling down on medium sized windows / screens produces weird sticky header

[miaulalala]

Steps to reproduce

1. Open Mail in a window that's not full size.
2. Open an email
3. Scroll down:
   

Expected behavior

The heading has an appropriate margin to the top bar

Actual behavior

The heading is partially hidden

Mail app version

4.0

Mailserver or service

gmx

Operating system

Ubuntu

PHP engine version

None

Web server

None

Database

None

Additional info

No response