Woowww @OSUblake,
Thanks a lot, it indeed does the trick!
edit : @OSUblake it is weird, when I look at it within the small codepen window, it works, but opening it in another window it doesnt, and I still have the same problem with some delay between scroll image and image within canvas .. When resizing the window, sometimes it works, but not always, so I think it is going closer to the solution, but I don't understand what is missing ?♂️