can't use flex inside neon-animated-pages #1694

I am using neon-animated-pages to animate the page. I have set it's height to 100%. I am using div inside it with background-image. But the height of div doesn't cover the entire browser height and is limited to it's content only. I have tried using flex with div but that also doesn't work. If I use this div outside of neon-animated-pages then it flexes well and cover the entire browser height.

What's going wrong inside neon-animated-pages?