This removes the env gate that opted into old behavior. Without
explicitly setting `scroll-behavior: 'smooth'` on the document, Next.js
will not attempt to programmatically disable smooth scrolling when
performing certain router events. This is a performance optimization in
all cases that do not care about smooth scroll.
**What:**
`layout-router` currently forces style recalculation on every navigation by manipulating the `scroll-behavior` CSS property on the `<html>` element. This would impact most navigations, even though most don't use smooth scrolling
**Why:**
To prevent smooth scrolling during router navigation (which can feel janky), Next.js temporarily resets `scroll-behavior` to `auto`, then restores the original value (#40642). While this solves the UX issue, it causes performance overhead for the vast majority of users who don't have smooth scrolling configured.
**How:**
This PR introduces an optimization that checks for a `data-scroll-behavior="smooth"` attribute before manipulating styles. Only users who explicitly opt into smooth scrolling will experience the style recalculation.
- The existing behavior is preserved by default
- Opt-in to the new behavior by setting `experimental.optimizeRouterScrolling: true` in Next.js config
- When smooth-scroll is detected on the document, a development warning is logged to notify that the default behavior will be changing in an upcoming major.
Zack Tanner