I think I’ve found a bug in the lightbox feature and wondered if this was something other people had encountered or could recreate.
In the lightbox page properties, if I set a border (inside or outside), when I preview or publish, the lightbox appears with (unwanted) scroll bars, but if I remove the border, the scroll bars disappear.
If I add a box that is an identical size to the lightbox, the scroll bars also aren’t there, which kind of resolves the issue, but just adds an extra element that shouldn’t really be required.