No longer possible to use two cms or console browser tabs open on *different* CMS applications

This used to work without problem with CMS 7.8/7.9 when using two instances of the CMS running on a different port.

With latest CMS trunk however (tested together with latest 7.9 version), even opening another browser tab (not yet logged in) already logs out the initial browser tab. And after logging in the other, any login/change in the initial window (or visa-versa) caused logout (session/cookie refresh?) on the other side.