element.scrollTo has had its operation change in 15.4 as well. Under 15.3, I could use a polyfill with scroll-behavior: smooth to scroll horizontally in a container with overflowx: hidden. Now with 15.4, I can't achieve that with or without a polyfill. My experimentation has found 2 disappointing almost-solutions but neither gets you back to the functionality you could achieve with a polyfill in 15.3. The options at present: The method you described: set the scroll-behavior to auto and get instant scroll with no scrollbar. If I set overflowx: auto, then scroll-behavior: smooth works as documented. If it's overflowx: hidden I get no movement at all. Neither one is great, and I really do hope this is a bug. (P.S. sorry to pile on your bug with something that's only mostly related - you're the only other person I've found talking about this and I'm so glad it's not just me).