I am having this issue on efpsa.org. First when I update to 4.3 from 3.5, our main menu was changed to a sidebar menu. We couldn’t change it back using the new customise option and the Widget area on the Dashbard is completely frozen with ‘Screen Options’ and ‘Help’ buttons misplaced and not working either. We deleted the sidebar menu and then the Main Menu automatically changed to the next sidebar menu that was alphabetically down the list. We made a workaround by re-naming our main menu so that it was first on the list alphabetically.
However, we cannot implement the sidebar menu we deleted and re-created because the Widget area is still frozen. And when we try to ‘Add Widget’ with the new customise option on the website it just adds the sidebar to every page on the website instead of the small group of pages we want it to appear on.
HALP.