refactor(vue-app): simplify scrollToTop checking (#8621)

packages/vue-app/template/router.scrollBehavior.js

@@ -22,36 +22,34 @@ if (process.client) {
   }
 }
 
+function shouldScrollToTop(route) {
+   const Pages = getMatchedComponents(route)
+   if (Pages.length === 1) {
+     const { options = {} } = Pages[0]
+     return options.scrollToTop !== false
+   }
+   return Pages.some(({ options }) => options && options.scrollToTop)
+}
+
 export default function (to, from, savedPosition) {
   // If the returned position is falsy or an empty object, will retain current scroll position
   let position = false
-
+  const isRouteChanged = to !== from
-  const Pages = getMatchedComponents(to)
-
-  // Scroll to the top of the page if...
-  if (
-      // One of the children set `scrollToTop`
-      (Pages.some(Page => Page.options && Page.options.scrollToTop) ||
-      // scrollToTop set in only page without children
-      (Pages.length < 2 && Pages.every(Page => !Page.options || Page.options.scrollToTop !== false))) &&
-      // route changes
-      to !== from
-  ) {
-    position = { x: 0, y: 0 }
-  }
 
   // savedPosition is only available for popstate navigations (back button)
   if (savedPosition) {
     position = savedPosition
+  } else if (isRouteChanged && shouldScrollToTop(to)) {
+    position = { x: 0, y: 0 }
   }
 
   const nuxt = window.<%= globals.nuxt %>
 
   if (
-    // Route hash changes
-    (to.path === from.path && to.hash !== from.hash) ||
     // Initial load (vuejs/vue-router#3199)
-    to === from
+    !isRouteChanged ||
+    // Route hash changes
+    (to.path === from.path && to.hash !== from.hash)
   ) {
     nuxt.$nextTick(() => nuxt.$emit('triggerScroll'))
   }