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.
It tuns out that this script was inserted as part of a server-wide hack. A Google search suggests that it’s only just appeared. As well as variably disrupting the loading page of a site, it sometimes also puts up a message “This webpage is using significant energy. Closing it may improve the responsiveness of your Mac.” The tech team behind my site have found that the script uses a visitor’s browser to mine Bitcoin! The are 1000s of lines of code that seem to involve some iterative maths procedure. So nothing to do with GP (phew!) but something to be alert to. Our site is not fixed yet: waiting on a full repair from the server end.