Simple solution: add this to the top of your code:  gsap.set(".sail", {y: 0, yPercent: -100}); Explanation: When you affect any transform-related values (x, y, xPercent, yPercent, scale, etc.) GSAP must first parse the CURRENT values which the browser always reports as a matrix() or matrix3d() which are pixel-based. It's impossible for GSAP to know that you intended those to be percent-based. So when GSAP reads (and caches) those values, it shoves transforms into the "y" and "x" comp
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