Hmm, full screen should take over the entire screen, but only on the same display. I think you're saying that you have two displays, and when you go full screen on one display, it also affects the ''other'' display where you expect to still be able to use the menu and dock. That is very strange and I can't recall seeing it mentioned before. I don't know whether this is a common issue with your version of MacOS or could be related to specific graphics hardware/driver software or settings. Hopefully other Mac users will have feedback on that.