Fix shallow routing scroll (#21437)

This ensures that Next.js will not scroll to the top of the page in shallow routing.

Fixes: https://github.com/vercel/next.js/issues/21386
Fixes: https://github.com/vercel/next.js/issues/21366

packages/next/next-server/lib/router/router.ts

@@ -1093,13 +1093,16 @@ export default class Router implements BaseRouter {
           !(routeInfo.Component as any).getInitialProps
       }
 
+      // shallow routing is only allowed for same page URL changes.
+      const isValidShallowRoute = options.shallow && this.route === route
       await this.set(
         route,
         pathname!,
         query,
         cleanedAs,
         routeInfo,
-        forcedScroll || (options.scroll ? { x: 0, y: 0 } : null)
+        forcedScroll ||
+          (isValidShallowRoute || !options.scroll ? null : { x: 0, y: 0 })
       ).catch((e) => {
         if (e.cancelled) error = error || e
         else throw e

test/integration/client-navigation/pages/nav/shallow-routing.js

@@ -27,6 +27,17 @@ export default class extends Component {
     Router.push(href, href, { shallow: true })
   }
 
+  increaseNonShallow() {
+    const counter = this.getCurrentCounter()
+    const href = `/nav/shallow-routing?counter=${counter + 1}`
+    Router.push(href, href, {})
+  }
+
+  gotoNavShallow() {
+    const href = `/nav`
+    Router.push(href, href, { shallow: true })
+  }
+
   render() {
     return (
       <div className="shallow-routing">
@@ -35,13 +46,21 @@ export default class extends Component {
             Home
           </a>
         </Link>
-        <div id="counter">Counter: {this.getCurrentCounter()}</div>
+        <div id="counter" style={{ marginBottom: 4000 }}>
+          Counter: {this.getCurrentCounter()}
+        </div>
         <div id="get-initial-props-run-count">
           getInitialProps run count: {this.props.getInitialPropsRunCount}
         </div>
         <button id="increase" onClick={() => this.increase()}>
           Increase
         </button>
+        <button id="increase2" onClick={() => this.increaseNonShallow()}>
+          Increase Non-Shallow
+        </button>
+        <button id="invalidShallow" onClick={() => this.gotoNavShallow()}>
+          Invalid Shallow Nav
+        </button>
       </div>
     )
   }

test/integration/client-navigation/test/index.test.js

@@ -854,6 +854,41 @@ describe('Client Navigation', () => {
 
       await browser.close()
     })
+
+    it('should keep the scroll position on shallow routing', async () => {
+      const browser = await webdriver(context.appPort, '/nav/shallow-routing')
+      await browser.eval(() =>
+        document.querySelector('#increase').scrollIntoView()
+      )
+      const scrollPosition = await browser.eval('window.pageYOffset')
+
+      expect(scrollPosition).toBeGreaterThan(3000)
+
+      await browser.elementByCss('#increase').click()
+      await waitFor(500)
+      const newScrollPosition = await browser.eval('window.pageYOffset')
+
+      expect(newScrollPosition).toBe(scrollPosition)
+
+      await browser.elementByCss('#increase2').click()
+      await waitFor(500)
+      const newScrollPosition2 = await browser.eval('window.pageYOffset')
+
+      expect(newScrollPosition2).toBe(0)
+
+      await browser.eval(() =>
+        document.querySelector('#invalidShallow').scrollIntoView()
+      )
+      const scrollPositionDown = await browser.eval('window.pageYOffset')
+
+      expect(scrollPositionDown).toBeGreaterThan(3000)
+
+      await browser.elementByCss('#invalidShallow').click()
+      await waitFor(500)
+      const newScrollPosition3 = await browser.eval('window.pageYOffset')
+
+      expect(newScrollPosition3).toBe(0)
+    })
   })
 
   describe('with URL objects', () => {