Add handling for prefetching onTouchStart and initial mobile testing (#38805)

This adds handling for prefetching `onTouchStart` as this gives a little more time to start parsing required scripts for a page transition if not already done that can help make the transition faster. This is based on research showing the touch start event firing on average `90ms` before click (x-ref: [source](https://instant.page/#:~:text=in%20the%20world.-,On%20mobile,-A%20user%20starts))

This also adds testing safari with playwright so we can run these in PRs instead of only after merge and adds initial mobile testing as well. 

x-ref: [slack thread](https://vercel.slack.com/archives/C7PDM7X2M/p1658250170774989?thread_ts=1658249275.178349&cid=C7PDM7X2M)

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm lint`
- [ ] The examples guidelines are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing.md#adding-examples)

.github/workflows/build_test_deploy.yml

@@ -816,13 +816,9 @@ jobs:
     runs-on: ubuntu-latest
     needs: [build, build-native-test]
     env:
-      BROWSERSTACK: true
       BROWSER_NAME: 'safari'
-      NEXT_TELEMETRY_DISABLED: 1
       NEXT_TEST_MODE: 'start'
-      SKIP_LOCAL_SELENIUM_SERVER: true
-      BROWSERSTACK_USERNAME: ${{ secrets.BROWSERSTACK_USERNAME }}
-      BROWSERSTACK_ACCESS_KEY: ${{ secrets.BROWSERSTACK_ACCESS_KEY }}
+      NEXT_TELEMETRY_DISABLED: 1
     steps:
       - name: Setup node
         uses: actions/setup-node@v3
@@ -851,12 +847,13 @@ jobs:
       - run: npm i -g pnpm@${PNPM_VERSION}
         if: ${{needs.build.outputs.docsChange == 'nope'}}
 
-      # TODO: use macos runner so that we can use playwright to test against
-      # PRs instead of only running on canary?
-      - run: '[[ -z "$BROWSERSTACK_ACCESS_KEY" ]] && echo "Skipping for PR" || npm i -g browserstack-local@1.4.0'
+      - run: npx playwright install-deps && npx playwright install webkit
         if: ${{needs.build.outputs.docsChange == 'nope'}}
 
-      - run: '[[ -z "$BROWSERSTACK_ACCESS_KEY" ]] && echo "Skipping for PR" || node run-tests.js -c 1 test/integration/production/test/index.test.js test/e2e/basepath.test.ts'
+      - run: node run-tests.js -c 1 test/integration/production/test/index.test.js test/e2e/basepath.test.ts
+        if: ${{needs.build.outputs.docsChange == 'nope'}}
+
+      - run: DEVICE_NAME='iPhone XR' node run-tests.js -c 1 test/production/prerender-prefetch/index.test.ts
         if: ${{needs.build.outputs.docsChange == 'nope'}}
 
   testSafariOld:

packages/next/client/link.tsx

@@ -48,6 +48,11 @@ type InternalLinkProps = {
    */
   onMouseEnter?: (e: any) => void
   // e: any because as it would otherwise overlap with existing types
+  /**
+   * requires experimental.newNextLinkBehavior
+   */
+  onTouchStart?: (e: any) => void
+  // e: any because as it would otherwise overlap with existing types
   /**
    * requires experimental.newNextLinkBehavior
    */
@@ -215,6 +220,7 @@ const Link = React.forwardRef<HTMLAnchorElement, LinkPropsReal>(
         locale: true,
         onClick: true,
         onMouseEnter: true,
+        onTouchStart: true,
         legacyBehavior: true,
       } as const
       const optionalProps: LinkPropsOptional[] = Object.keys(
@@ -239,7 +245,11 @@ const Link = React.forwardRef<HTMLAnchorElement, LinkPropsReal>(
               actual: valType,
             })
           }
-        } else if (key === 'onClick' || key === 'onMouseEnter') {
+        } else if (
+          key === 'onClick' ||
+          key === 'onMouseEnter' ||
+          key === 'onTouchStart'
+        ) {
           if (props[key] && valType !== 'function') {
             throw createPropError({
               key,
@@ -296,6 +306,7 @@ const Link = React.forwardRef<HTMLAnchorElement, LinkPropsReal>(
       locale,
       onClick,
       onMouseEnter,
+      onTouchStart,
       legacyBehavior = Boolean(process.env.__NEXT_NEW_LINK_BEHAVIOR) !== true,
       ...restProps
     } = props
@@ -411,6 +422,7 @@ const Link = React.forwardRef<HTMLAnchorElement, LinkPropsReal>(
     }, [as, href, isVisible, locale, p, router])
 
     const childProps: {
+      onTouchStart: React.TouchEventHandler
       onMouseEnter: React.MouseEventHandler
       onClick: React.MouseEventHandler
       href?: string
@@ -466,6 +478,23 @@ const Link = React.forwardRef<HTMLAnchorElement, LinkPropsReal>(
           prefetch(router, href, as, { priority: true })
         }
       },
+      onTouchStart: (e: React.TouchEvent<HTMLAnchorElement>) => {
+        if (!legacyBehavior && typeof onTouchStart === 'function') {
+          onTouchStart(e)
+        }
+
+        if (
+          legacyBehavior &&
+          child.props &&
+          typeof child.props.onTouchStart === 'function'
+        ) {
+          child.props.onTouchStart(e)
+        }
+
+        if (isLocalURL(href)) {
+          prefetch(router, href, as, { priority: true })
+        }
+      },
     }
 
     // If child is an <a> tag and doesn't have a href attribute, or if the 'passHref' property is

test/integration/production/test/index.test.js

@@ -750,7 +750,7 @@ describe('Production Usage', () => {
         .elementByCss('a')
         .click()
         .waitForElementByCss('.about-page')
-        .elementByCss('div')
+        .elementByCss('.about-page')
         .text()
 
       expect(text).toBe('About Page')

test/lib/browsers/base.ts

@@ -31,6 +31,9 @@ export class BrowserInterface {
   elementById(selector: string): BrowserInterface {
     return this
   }
+  touchStart(): BrowserInterface {
+    return this
+  }
   click(opts?: { modifierKey?: boolean }): BrowserInterface {
     return this
   }

test/lib/browsers/playwright.ts

@@ -8,6 +8,7 @@ import {
   BrowserContext,
   Page,
   ElementHandle,
+  devices,
 } from 'playwright-chromium'
 import path from 'path'
 
@@ -49,6 +50,17 @@ class Playwright extends BrowserInterface {
   async setup(browserName: string, locale?: string) {
     if (browser) return
     const headless = !!process.env.HEADLESS
+    let device
+
+    if (process.env.DEVICE_NAME) {
+      device = devices[process.env.DEVICE_NAME]
+
+      if (!device) {
+        throw new Error(
+          `Invalid playwright device name ${process.env.DEVICE_NAME}`
+        )
+      }
+    }
 
     if (browserName === 'safari') {
       browser = await webkit.launch({ headless })
@@ -57,7 +69,7 @@ class Playwright extends BrowserInterface {
     } else {
       browser = await chromium.launch({ headless, devtools: !headless })
     }
-    context = await browser.newContext({ locale })
+    context = await browser.newContext({ locale, ...device })
   }
 
   async get(url: string): Promise<void> {
@@ -284,6 +296,12 @@ class Playwright extends BrowserInterface {
     })
   }
 
+  touchStart() {
+    return this.chain((el: ElementHandle) => {
+      return el.dispatchEvent('touchstart').then(() => el)
+    })
+  }
+
   elementsByCss(sel) {
     return this.chain(() =>
       page.$$(sel).then((els) => {

test/production/prerender-prefetch/index.test.ts

@@ -134,43 +134,83 @@ describe('Prerender prefetch', () => {
     expect(isNaN(newTime)).toBe(false)
   })
 
-  it('should attempt cache update on link hover', async () => {
-    const browser = await webdriver(next.url, '/')
-    const timeRes = await fetchViaHTTP(
-      next.url,
-      `/_next/data/${next.buildId}/blog/first.json`,
-      undefined,
-      {
-        headers: {
-          purpose: 'prefetch',
-        },
-      }
-    )
-    const startTime = (await timeRes.json()).pageProps.now
+  if (process.env.DEVICE_NAME) {
+    it('should attempt cache update on touchstart', async () => {
+      const browser = await webdriver(next.url, '/')
+      const timeRes = await fetchViaHTTP(
+        next.url,
+        `/_next/data/${next.buildId}/blog/first.json`,
+        undefined,
+        {
+          headers: {
+            purpose: 'prefetch',
+          },
+        }
+      )
+      const startTime = (await timeRes.json()).pageProps.now
 
-    // ensure stale data is used by default
-    await browser.elementByCss('#to-blog-first').click()
-    await check(() => browser.elementByCss('#page').text(), 'blog/[slug]')
+      // ensure stale data is used by default
+      await browser.elementByCss('#to-blog-first').click()
+      await check(() => browser.elementByCss('#page').text(), 'blog/[slug]')
 
-    expect(JSON.parse(await browser.elementByCss('#props').text()).now).toBe(
-      startTime
-    )
-    await browser.back().waitForElementByCss('#to-blog-first')
-    const requests = []
+      expect(JSON.parse(await browser.elementByCss('#props').text()).now).toBe(
+        startTime
+      )
+      await browser.back().waitForElementByCss('#to-blog-first')
+      const requests = []
 
-    browser.on('request', (req) => {
-      requests.push(req.url())
+      browser.on('request', (req) => {
+        requests.push(req.url())
+      })
+
+      // now trigger cache update and navigate again
+      await check(async () => {
+        await browser.elementByCss('#to-blog-second').touchStart()
+        await browser.elementByCss('#to-blog-first').touchStart()
+        return requests.some((url) => url.includes('/blog/first.json'))
+          ? 'success'
+          : requests
+      }, 'success')
     })
+  } else {
+    it('should attempt cache update on link hover', async () => {
+      const browser = await webdriver(next.url, '/')
+      const timeRes = await fetchViaHTTP(
+        next.url,
+        `/_next/data/${next.buildId}/blog/first.json`,
+        undefined,
+        {
+          headers: {
+            purpose: 'prefetch',
+          },
+        }
+      )
+      const startTime = (await timeRes.json()).pageProps.now
 
-    // now trigger cache update and navigate again
-    await check(async () => {
-      await browser.elementByCss('#to-blog-second').moveTo()
-      await browser.elementByCss('#to-blog-first').moveTo()
-      return requests.some((url) => url.includes('/blog/first.json'))
-        ? 'success'
-        : requests
-    }, 'success')
-  })
+      // ensure stale data is used by default
+      await browser.elementByCss('#to-blog-first').click()
+      await check(() => browser.elementByCss('#page').text(), 'blog/[slug]')
+
+      expect(JSON.parse(await browser.elementByCss('#props').text()).now).toBe(
+        startTime
+      )
+      await browser.back().waitForElementByCss('#to-blog-first')
+      const requests = []
+
+      browser.on('request', (req) => {
+        requests.push(req.url())
+      })
+
+      // now trigger cache update and navigate again
+      await check(async () => {
+        await browser.elementByCss('#to-blog-second').moveTo()
+        await browser.elementByCss('#to-blog-first').moveTo()
+        return requests.some((url) => url.includes('/blog/first.json'))
+          ? 'success'
+          : requests
+      }, 'success')
+    })
+  }
 
   it('should handle failed data fetch and empty cache correctly', async () => {
     const browser = await webdriver(next.url, '/')