Overnight (ie. without me doing anything!), the behaviour of the primary navigation bar has changed in a strange way. The navigation bar background takes up the whole screen when the page is scrolled to the top. Any menu item that is selected also extends the full depth of the screen. Once I scroll down a screen’s depth, everything behaves normally. Trying other navigation menu layouts, such as shifting it to a sidebar also shows the same behaviour: the navigation background extends to the bottom of the screen until it is scrolled down.
Until now, the site has been working fine. I’ve deactivated all the plug-ins, except GP and the issue is till there. I’ve tried a different theme (Twenty Seventeen) and the menus work fine there.