Dave, as you requested, I have posted the details of my problem here.
Sadly, I’m going crazy with this. I have been watching the video on recreating a Muse web page (with the parachute). The issue I have is with the scrolling clouds, which I cannot get to work properly. If I try to apply the movement to the clouds container (that contains the background image), either nothing happens, or everything moves all over the place (past the right and left margins causing the page to scroll horizontally) - including the light box, which, according to the video, is stationary.
Think about it, you are targeting the container that contains all other elements and that’s why everything is moving. You need to use a separate div for the bg image with position set to absolute and target only that div.
I did my best to follow your video. Sadly I keep getting lost - my file appears to be out of sync with the state it is when your video starts. I’m sorry to trouble you with this. Can you go through the steps around 50% slower, with the current file? Thank you so much.