That's so you don't accidentally leave the item form and cause the item to lock. The behaviour is the same across all edit forms.

This is indeed a bug but thankfully it appears only when you swap categories (and thus load a different set of extra fields). I'll sort it out in the coming days.

As for the sidebar, the UI refresh does not stop here. An update will follow in a matter of days improving its responsiveness and adding an option to minimize the sidebar into an icons column only.

Fixed in K2 v2.10.1 (dev). If you need to update now, use: github.com/getk2/k2/archive/master.zip

The final version should be out by Friday tops.

Not yet, but that's coming too. Updates will be more frequent from now on either way.