I figured out one part of this. It turns out that the spin works on first load if I do it as an “WebPage/iFrame” content. Previously I’d had it loading in a “Stacks” content wrapper, using and HTML stack within it. That had the advantage of allowing me to include other content in the Lightbox, like a caption. But if I give up on that and use your WebPage/iFrame content option, it loads perfectly and spins on the first load.