fix($core): SSR error when async components were registered in runtime.

packages/@vuepress/core/lib/app/components/Content.js

@@ -15,8 +15,14 @@ export default {
 
   render (h, { parent, props, data }) {
     const pageKey = props.pageKey || parent.$page.key
+
     if (components[pageKey]) {
-      Vue.component(pageKey, components[pageKey])
+      // In SSR, if a component is not registered with the component option
+      // vue-server-renderer will not be able to resovle it.
+      if (!parent.$ssrContext) {
+        Vue.component(pageKey, components[pageKey])
+      }
+
       return h(pageKey, {
         class: [props.custom ? 'custom' : '', data.class, data.staticClass],
         style: data.style,

packages/@vuepress/core/lib/app/serverEntry.js

+import Vue from 'vue'
 import { createApp } from './app'
+import pageComponents from '@internal/page-components'
+import layoutComponents from '@internal/layout-components'
 
 export default context => new Promise((resolve, reject) => {
   const { app, router } = createApp(true /* isServer */)
@@ -10,5 +13,26 @@ export default context => new Promise((resolve, reject) => {
   }
 
   router.push(url)
-  router.onReady(() => resolve(app))
+
+  // In SSR, if a component is not registered with the component option,
+  // vue-server-renderer will not able to resolve it.
+  //
+  // Build also works after deleting this, but the content of all pages
+  // will not appear to the output html, which is not conducive to SEO.
+  const asyncComponentLoadingPromises = [
+    ...getComponentArr(pageComponents),
+    ...getComponentArr(layoutComponents)
+  ].map(({ name, loadFn }) => {
+    return loadFn.then(comp => {
+      Vue.component(name, comp.default)
+    })
+  })
+
+  router.onReady(() => {
+    Promise.all(asyncComponentLoadingPromises).then(() => resolve(app))
+  })
 })
+
+function getComponentArr (components) {
+  return Object.keys(components).map(name => ({ name, loadFn: components[name] }))
+}