@GreenSock
In your example above from codepen if you change the yPercent to xPercent you get the effect of right to left instead of down to up, then I assumed if you changed xPercent 1 ? 100 : 0 to xPercent 1 ? 0 : 100 but that doesn't seem to work.
panels.forEach((panel,i)=>{
// first panel should start already in place, and last panel should end in the center.
tl.fromTo(panel, {
xPercent: i ? 100 : 0
},{
xPercent: i === panels.length - 1 ? 0 : -100,
duration: i === 0 || i === panels.length - 1 ? 0.5 : 1,
ease: "none",
}, "-=0.5");
});