fix: allow smooth scrolling if only hash changes (pages & app) (#52915)

We were preventing smooth scrolling to avoid jarring UX when `scroll-behavior: smooth` was set and the user navigates to another route ([PR](#40642) and [related comment](#51721 (comment))). 

This updates both pages & app router to restore smooth scroll functionality if the only the route hash changes.

Fixes #51721
Closes NEXT-1406

packages/next/src/client/components/layout-router.tsx

@@ -158,7 +158,7 @@ interface ScrollAndFocusHandlerProps {
   segmentPath: FlightSegmentPath
 }
 class InnerScrollAndFocusHandler extends React.Component<ScrollAndFocusHandlerProps> {
-  handlePotentialScroll = () => {
+  handlePotentialScroll = (isUpdate?: boolean) => {
     // Handle scroll and focus, it's only applied once in the first useEffect that triggers that changed.
     const { focusAndScrollRef, segmentPath } = this.props
 
@@ -247,6 +247,7 @@ class InnerScrollAndFocusHandler extends React.Component<ScrollAndFocusHandlerPr
         {
           // We will force layout by querying domNode position
           dontForceLayout: true,
+          onlyHashChange: !!isUpdate,
         }
       )
 
@@ -262,7 +263,7 @@ class InnerScrollAndFocusHandler extends React.Component<ScrollAndFocusHandlerPr
   componentDidUpdate() {
     // Because this property is overwritten in handlePotentialScroll it's fine to always run it when true as it'll be set to false for subsequent renders.
     if (this.props.focusAndScrollRef.apply) {
-      this.handlePotentialScroll()
+      this.handlePotentialScroll(true)
     }
   }
 

packages/next/src/shared/lib/router/router.ts

@@ -2247,27 +2247,35 @@ export default class Router implements BaseRouter {
 
   scrollToHash(as: string): void {
     const [, hash = ''] = as.split('#')
-    // Scroll to top if the hash is just `#` with no value or `#top`
-    // To mirror browsers
-    if (hash === '' || hash === 'top') {
-      handleSmoothScroll(() => window.scrollTo(0, 0))
-      return
-    }
 
-    // Decode hash to make non-latin anchor works.
-    const rawHash = decodeURIComponent(hash)
-    // First we check if the element by id is found
-    const idEl = document.getElementById(rawHash)
-    if (idEl) {
-      handleSmoothScroll(() => idEl.scrollIntoView())
-      return
-    }
-    // If there's no element with the id, we check the `name` property
-    // To mirror browsers
-    const nameEl = document.getElementsByName(rawHash)[0]
-    if (nameEl) {
-      handleSmoothScroll(() => nameEl.scrollIntoView())
-    }
+    handleSmoothScroll(
+      () => {
+        // Scroll to top if the hash is just `#` with no value or `#top`
+        // To mirror browsers
+        if (hash === '' || hash === 'top') {
+          window.scrollTo(0, 0)
+          return
+        }
+
+        // Decode hash to make non-latin anchor works.
+        const rawHash = decodeURIComponent(hash)
+        // First we check if the element by id is found
+        const idEl = document.getElementById(rawHash)
+        if (idEl) {
+          idEl.scrollIntoView()
+          return
+        }
+        // If there's no element with the id, we check the `name` property
+        // To mirror browsers
+        const nameEl = document.getElementsByName(rawHash)[0]
+        if (nameEl) {
+          nameEl.scrollIntoView()
+        }
+      },
+      {
+        onlyHashChange: this.onlyAHashChange(as),
+      }
+    )
   }
 
   urlIsNew(asPath: string): boolean {

packages/next/src/shared/lib/router/utils/handle-smooth-scroll.ts

@@ -4,8 +4,14 @@
  */
 export function handleSmoothScroll(
   fn: () => void,
-  options: { dontForceLayout?: boolean } = {}
+  options: { dontForceLayout?: boolean; onlyHashChange?: boolean } = {}
 ) {
+  // if only the hash is changed, we don't need to disable smooth scrolling
+  // we only care to prevent smooth scrolling when navigating to a new page to avoid jarring UX
+  if (options.onlyHashChange) {
+    fn()
+    return
+  }
   const htmlElement = document.documentElement
   const existing = htmlElement.style.scrollBehavior
   htmlElement.style.scrollBehavior = 'auto'