An image lightbox from scratch: a centered overlay is easy — the real work is scroll-lock, focus-trap, and returning focus

A lightbox is the pattern you meet on every gallery, product page and image feed — click a thumbnail, the photo blows up into a fullscreen overlay over a dimmed page. I built a complete one with no library: prev/next with wraparound, a 3 / 8 counter, zoom, keyboard nav, and the accessibility work most home-made modals skip. The layout took ten minutes. Everything worth writing about is the behaviour around the image. Here it is. The whole thing is a single position:fixed element pinned to all fo