Inertial scrolling

Touch and trackpad scrolling already has inertia built in: you flick, the page keeps going and decays to a stop. What people usually mean by this term on the web is the authored version, better called scroll smoothing: JavaScript keeps a target scroll position and eases the actual position toward it every frame (a lerp), so wheel and keyboard input get that same glide, tuned heavier than the platform default. That extra weight is most of what makes Awwwards-style sites feel expensive.

In practice you reach for a library: Lenis is the current default, Locomotive Scroll the older one, and GSAP's ScrollSmoother the paid option. Two knobs do the work: the lerp factor (how much of the gap you close per frame, roughly 0.05 to 0.12) and the wheel multiplier. Lower lerp equals heavier glide. Anything under about 0.04 stops feeling smooth and starts feeling broken.

Gotchas, and there are real ones. The old transform-based smoothers (classic Locomotive) translated a wrapper instead of scrolling the page, so the visual position disagreed with the browser's real scroll offset and sticky positioning, IntersectionObserver, and anchor links all went wrong. Lenis-style libraries drive the native scroll position instead, so most of that keeps working, but you still hand the library the rAF loop and route anchor jumps through its own scrollTo. Either way, find-in-page and keyboard scrolling feel sluggish, some people get motion sick, and trackpad users already have momentum, so you are stacking easing on easing. Always kill it under 'prefers-reduced-motion'.