[WIP] Remember scroll position on error (vercel#911)

* Remember scroll position on error

* Added comment + check if lastScroll was set

* Remove check for lastAppProps

* Use events to make scroll persistence dev-only

* Return EventEmitter from next()

* Update next-dev.js

client/index.js

@@ -1,5 +1,6 @@
 import { createElement } from 'react'
 import ReactDOM from 'react-dom'
+import { EventEmitter } from 'events'
 import HeadManager from './head-manager'
 import { rehydrate } from '../lib/css'
 import { createRouter } from '../lib/router'
@@ -33,13 +34,16 @@ const headManager = new HeadManager()
 const container = document.getElementById('__next')
 
 export default (onError) => {
+  const emitter = new EventEmitter()
   if (ids && ids.length) rehydrate(ids)
 
   router.subscribe(({ Component, props, err }) => {
-    render({ Component, props, err }, onError)
+    render({ Component, props, err, emitter }, onError)
   })
 
-  render({ Component, props, err }, onError)
+  render({ Component, props, err, emitter }, onError)
+
+  return emitter
 }
 
 export async function render (props, onError = renderErrorComponent) {
@@ -56,7 +60,7 @@ async function renderErrorComponent (err) {
   await doRender({ Component: ErrorComponent, props, err })
 }
 
-async function doRender ({ Component, props, err }) {
+async function doRender ({ Component, props, err, emitter }) {
   if (!props && Component &&
     Component !== ErrorComponent &&
     lastAppProps.Component === ErrorComponent) {
@@ -65,10 +69,19 @@ async function doRender ({ Component, props, err }) {
     props = await loadGetInitialProps(Component, { err, pathname, query })
   }
 
+  if (emitter) {
+    emitter.emit('before-reactdom-render', { Component })
+  }
+
   Component = Component || lastAppProps.Component
   props = props || lastAppProps.props
 
   const appProps = { Component, props, err, router, headManager }
-  lastAppProps = appProps
   ReactDOM.render(createElement(App, appProps), container)
+
+  if (emitter) {
+    emitter.emit('after-reactdom-render', { Component })
+  }
+
+  lastAppProps = appProps
 }

client/next-dev.js

@@ -1,4 +1,8 @@
 import patch from './patch-react'
+import evalScript from '../lib/eval-script'
+
+const { __NEXT_DATA__: { errorComponent } } = window
+const ErrorComponent = evalScript(errorComponent).default
 
 // apply patch first
 patch((err) => {
@@ -13,10 +17,32 @@ require('react-hot-loader/patch')
 
 const next = window.next = require('./')
 
-next.default(onError)
+const emitter = next.default(onError)
 
 function onError (err) {
   // just show the debug screen but don't render ErrorComponent
   // so that the current component doesn't lose props
-  next.render({ err })
+  next.render({ err, emitter })
 }
+
+let lastScroll
+
+emitter.on('before-reactdom-render', ({ Component }) => {
+  // Remember scroll when ErrorComponent is being rendered to later restore it
+  if (!lastScroll && Component === ErrorComponent) {
+    const { pageXOffset, pageYOffset } = window
+    lastScroll = {
+      x: pageXOffset,
+      y: pageYOffset
+    }
+  }
+})
+
+emitter.on('after-reactdom-render', ({ Component }) => {
+  if (lastScroll && Component !== ErrorComponent) {
+    // Restore scroll after ErrorComponent was replaced with a page component by HMR
+    const { x, y } = lastScroll
+    window.scroll(x, y)
+    lastScroll = null
+  }
+})