One of the core principles of macOS is that an app can have multiple windows. This makes it easy to read something in a browser window, while at the same time writing something in another one. Unless you are using GitLab and someone decides they have to artificially dim one window so you can’t read anything there any more. Safari and they decide that two pinned tabs shouldn’t be used at the same time.

Why?

Update 2020-07-15: Attributed error to Safari not Gitlab. Thanks @Cisneiros