stackoverflow and meta.stackoverflow sometimes have different UI code than the other SE sites. IIRC they run on a separate server too.
The header of this site is not fixed, because _________ (fill in the blank). They're probably in the middle of a UI update and only wanted to test it on stackoverflow.com until it's ready to go for the other sites, but that'...
What you are seeing there is the mobile view.
If you scroll down to the very bottom of the page you will see a link that says "full site" which will set a cookie in your browser to give you the desktop version of the site:
As to why your browser suddenly switched to that version I'm not sure. Are you able to post your useragent string? You can get it ...
Stack Overflow is essentially running "updated" Stack Exchange software.
they have updated profile pages
they have "teams" where groups of users can collaborate as companies or projects
they have a "developer jobs" sub-site
they have a very different top bar, where the site logo is integrated rather than below it as with other sites
There are many ...
I'll try and address your issues individually here:
As I stated, this new box requires a click to invoke and a click to dismiss, making it more work than the previous one.
This is intentional, the hover behavior was not intuitive. We do however enable hovering once you click to open any of the buttons on the left, since you have declared your intent.