Viewing stackoverflow.com HTTPS and clicking on a notification link for the meta site changes from HTTPS to HTTP. The sites should remain in whatever protocol the user is currently using when switching from site to site.
This is not a bug, HTTPS support outside of login is not supported yet. We have to solve the drastic performance problems that HTTPS brings before opening the gates on that one - which we're close to being able to do. Then, we'll address the mixed content issues - but those don't matter if we can't get it to perform decently, so they come second on the priority list.
When we enable HTTPS, we'll do so via HSTS for navigation. This will give you the option of security given the performance difference inherent in the handshakes needed for TLS.