I took another look at this, and I’m making some progress.
I have the ‘More’ menu expanding back out on resize to a larger window, and I have the ‘More’ sub-menus partially working – they work when the page is loaded at a display size that makes the ‘More’ menu appear from the outset, but they don’t work properly yet when the window is resized down.
More news in the next day or so, and I’ll share some code.
Ben