minor: Improve scrollBehavior to work with transitions, resolve #1376, thanks to @homerjam

lib/app/components/nuxt-child.js

@@ -29,6 +29,12 @@ export default {
         listeners[key] = transition[key].bind(_parent)
       }
     })
+    // Add triggerScroll event on beforeEnter (fix #1376)
+    let beforeEnter = listeners.beforeEnter
+    listeners.beforeEnter = (el) => {
+      window.$nuxt.$emit('triggerScroll')
+      if (beforeEnter) return beforeEnter.call(_parent, el)
+    }
 
     let routerView = [
       h('router-view', data)

lib/app/router.js

@@ -26,27 +26,40 @@ uniqBy(_components, '_name').forEach((route) => { %>const <%= route._name %> = (
 <% if (router.scrollBehavior) { %>
 const scrollBehavior = <%= serialize(router.scrollBehavior).replace('scrollBehavior(', 'function(').replace('function function', 'function') %>
 <% } else { %>
-const scrollBehavior = (to, from, savedPosition) => {
-  // SavedPosition is only available for popstate navigations.
-  if (savedPosition) {
-    return savedPosition
-  } else {
-    let position = {}
-    // If no children detected
-    if (to.matched.length < 2) {
-      // Scroll to the top of the page
-      position = { x: 0, y: 0 }
-    }
-    else if (to.matched.some((r) => r.components.default.options.scrollToTop)) {
-      // If one of the children has scrollToTop option set to true
-      position = { x: 0, y: 0 }
-    }
-    // If link has anchor, scroll to anchor by returning the selector
-    if (to.hash) {
-      position = { selector: to.hash }
-    }
-    return position
+if (process.client) {
+  window.history.scrollRestoration = 'manual'
+}
+const scrollBehavior = function (to, from, savedPosition) {
+  // if the returned position is falsy or an empty object,
+  // will retain current scroll position.
+  let position = false
+
+  // if no children detected
+  if (to.matched.length < 2) {
+    // scroll to the top of the page
+    position = { x: 0, y: 0 }
+  } else if (to.matched.some((r) => r.components.default.options.scrollToTop)) {
+    // if one of the children has scrollToTop option set to true
+    position = { x: 0, y: 0 }
   }
+
+  // savedPosition is only available for popstate navigations (back button)
+  if (savedPosition) {
+    position = savedPosition
+  }
+
+  return new Promise(resolve => {
+    // wait for the out transition to complete (if necessary)
+    window.$nuxt.$once('triggerScroll', () => {
+      // coords will be used if no selector is provided,
+      // or if the selector didn't match any element.
+      if (to.hash && document.querySelector(to.hash)) {
+        // scroll to anchor by returning the selector
+        position = { selector: to.hash }
+      }
+      resolve(position)
+    })
+  })
 }
 <% } %>