Change the position: absolute
on your .content
div to position: relative
.
The absolute positioning of it takes it (and thus the entire page's contents) out of the flow, collapsing the body
down to a height of 0. You can verify that by checking e.g. document.body.scrollHeight
.
Since Lightbox then sees the body has no height, it only bothers adding the overlay to one screen-full.
Edit: As a work-around for the problem noted in the comments (the overlay still doesn't cover everything when the lightbox extends below the bottom of the original page), you could add some extra padding to the bottom of the page.