[WIP] Remember scroll position on error

client/index.js

@@ -8,6 +8,7 @@ import evalScript from '../lib/eval-script'
 import { loadGetInitialProps } from '../lib/utils'
 
 const {
+  CustomEvent,
   __NEXT_DATA__: {
     component,
     errorComponent,
@@ -65,10 +66,23 @@ async function doRender ({ Component, props, err }) {
     props = await loadGetInitialProps(Component, { err, pathname, query })
   }
 
+  // Try/catch is needed because IE11 has a CustomEvent implementation without contructor
+  try {
+    const event = new CustomEvent('before-reactdom-render', { detail: { Component } })
+    document.dispatchEvent(event)
+  } catch (e) {}
+
   Component = Component || lastAppProps.Component
   props = props || lastAppProps.props
 
   const appProps = { Component, props, err, router, headManager }
-  lastAppProps = appProps
   ReactDOM.render(createElement(App, appProps), container)
+
+  // Try/catch is needed because IE11 has a CustomEvent implementation without contructor
+  try {
+    const event = new CustomEvent('after-reactdom-render', { detail: { Component } })
+    document.dispatchEvent(event)
+  } catch (e) {}
+
+  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) => {
@@ -20,3 +24,25 @@ function onError (err) {
   // so that the current component doesn't lose props
   next.render({ err })
 }
+
+let lastScroll
+
+document.addEventListener('before-reactdom-render', ({ detail: { 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
+    }
+  }
+})
+
+document.addEventListener('after-reactdom-render', ({ detail: { 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 = false
+  }
+})