Okay, I discovered that this way of horizontal scrolling doesn't work in Firefox so I built it again using ScrollTrigger for the horizontal scroll, exactly like here: https://codepen.io/GreenSock/pen/YzygYvM
I expected the animation of the sidebar to work with start: () => `top+=${innerWidth * 0.5} center`, etc. but it somehow doesn't. With the regular start: 'top center', and variations it's also not behaving as it should. What's the trick here?