Prevent subdomain breakout to separate tab for pinned tab

[loftux]

Web app that use multiple subdomains after login that you want to keep using inside the one pinned tab.

Example
You have pinned startpage.example.com, for login it goes to login.example.com, and when logged in it redirects to random app1.example.com, app2.example.com and so on.
You want all of this redirects to different subdomains stay within the pinned tab that match the core domain, but now it breaks free to a separate non-pinned tab.

If you do the same for a non-pinned tab, all redirects are within the same tab, it is just when pinned it breaks free, and you end up with more than one tab for the webb app you intend to use.

This feature would be dependent/enhanced if #2221, allow to edit the url for a pinned tab, is implemented first. This so you can properly set the base domain.