Add Global API Changes migration page

src/.vuepress/config.js

@@ -55,7 +55,10 @@ const sidebar = {
     {
       title: 'Tooling',
       collapsable: false,
-      children: ['/guide/single-file-component']
+      children: [
+        '/guide/single-file-component',
+        '/guide/testing'
+      ]
     },
     {
       title: 'Scaling Up',
@@ -67,15 +70,14 @@ const sidebar = {
         '/guide/accessibility'
       ]
     },
-    {
-      title: 'Tooling',
-      collapsable: false,
-      children: ['/guide/testing']
-    },
     {
       title: 'Migration to Vue 3',
       collapsable: true,
-      children: ['migration', 'migration/functional-components']
+      children: [
+        'migration/global-api',
+        'migration/treeshaking',
+        'migration/functional-components'
+      ]
     },
     {
       title: 'Contribute to the Docs',

src/.vuepress/styles/index.styl

@@ -17,7 +17,6 @@
   position: relative;
   border-bottom-right-radius: 2px;
   border-top-right-radius: 2px;
-  max-width: 80%;
   margin: 1rem 0;
 
   &::before {

src/guide/migration/global-api.md

@@ -0,0 +1,204 @@
+# Global API Changes
+
+Vue 2.x has a number of global APIs and configurations that globally mutate Vue’s behavior. For instance, to create a global component, you would use the `Vue.component` API like this:
+
+``` js
+Vue.component('button-counter', {
+  data: () => ({
+    count: 0
+  }),
+  template: '<button @click="count++">Clicked {{ count }} times.</button>'
+})
+```
+
+Similarly, this is how a global directive is declared:
+
+``` js
+Vue.directive('focus', {
+  inserted: el => el.focus()
+})
+```
+
+While this approach is convenient, it leads to a couple of problems. Technically, Vue 2 doesn't have a concept of an "app". What we define as an app is simply a root Vue instance created via `new Vue()`. Every root instance created from the same Vue constructor **shares the same global configuration**. As a result:
+
+* Global configuration makes it easy to accidentally pollute other test cases during testing. Users need to carefully store original global configuration and restore it after each test (e.g. resetting `Vue.config.errorHandler`). Some APIs like `Vue.use` and `Vue.mixin` don't even have a way to revert their effects. This makes tests involving plugins particularly tricky. In fact, vue-test-utils has to implement a special API `createLocalVue` to deal with this:
+
+	``` js
+  import { createLocalVue, mount } from '@vue/test-utils'
+
+  // create an extended `Vue` constructor
+  const localVue = createLocalVue()
+
+  // install a plugin “globally” on the “local” Vue constructor
+  localVue.use(MyPlugin)
+
+  // pass the `localVue` to the mount options
+  mount(Component, { localVue })
+	```
+
+* Global configuration makes it difficult to share the same copy of Vue between multiple "apps" on the same page, but with different global configurations. 
+
+  ```js
+  // this affects both root instances
+  Vue.mixin({ /* ... */ })
+
+  const app1 = new Vue({ el: '#app-1' })
+  const app2 = new Vue({ el: '#app-2' })
+  ```
+
+To avoid these problems, in Vue 3 we introduce… 
+
+## A New Global API: `createApp`
+
+Calling `createApp` returns an _app instance_, a new concept in Vue 3.
+
+```js
+import { createApp } from 'vue'
+
+const app = createApp() 
+```
+
+An app instance exposes a subset of the current global APIs. The rule of thumb is _any APIs that globally mutate Vue's behavior are now moved to the app instance_. Here is a table of the current global APIs and their corresponding instance APIs:
+
+| 2.x Global API | 3.x Instance API (`app`) |
+|----------------|--------------------------|
+| Vue.config | app.config |
+| Vue.config.productionTip | _removed_ ([see below](config-productiontip-removed)) |
+| Vue.config.ignoredElements | app.config.isCustomElement ([see below](#config-ignoredelements-is-now-config-iscustomelement)) |
+| Vue.component | app.component |
+| Vue.directive | app.directive |
+| Vue.mixin | app.mixin |
+| Vue.use | app.use ([see below](#a-note-for-plugin-authors)) |
+
+All other global APIs that do not globally mutate behavior are now named exports, as documented in [Global API Treeshaking](./treeshaking.html).
+
+### `config.productionTip` Removed
+
+In Vue 3.x, the "use production build" tip will only show up when using the "dev + full build" (the build that includes the runtime compiler and has warnings).
+
+For ES modules builds, since they are used with bundlers, and in most cases a CLI or boilerplate would have configured the production env properly, this tip will no longer show up.
+
+### `config.ignoredElements` Is Now `config.isCustomElement`
+
+This config option was introduced with the intention to support native custom elements, so the renaming better conveys what it does. The new option also expects a function which provides more flexibility than the old string / RegExp approach:
+
+``` js
+// before
+Vue.config.ignoredElements = ['my-el', /^ion-/]
+
+// after
+const app = Vue.createApp()
+app.config.isCustomElement = tag => tag.startsWith('ion-')
+```
+
+::: tip Important
+
+In 3.0, the check of whether an element is a component or not has been moved to the template compilation phase, therefore this config option is only respected when using the runtime compiler. If you are using the runtime-only build, `isCustomElement` must be passed to `@vue/compiler-dom` in the build setup instead - for example, via the [`compilerOptions` option in vue-loader](https://vue-loader.vuejs.org/options.html#compileroptions).
+
+* If `config.isCustomElement` is assigned to when using a runtime-only build, a warning will be emitted instructing the user to pass the option in the build setup instead;
+* This will be a new top-level option in the Vue CLI config.
+::: 
+
+### A Note for Plugin Authors
+
+It is a common practice for plugin authors to install the plugins automatically in their UMD builds using `Vue.use`. For instance, this is how the official `vue-router` plugin installs itself in a browser environment:
+
+```js
+var inBrowser = typeof window !== 'undefined';
+/* … */
+if (inBrowser && window.Vue) {
+  window.Vue.use(VueRouter);
+}
+```
+
+As the `use` global API is no longer available in Vue 3, this method will cease to work and calling `Vue.use()` will now trigger a warning. Instead, the end-user will now have to explicitly specify using the plugin on the app instance:
+
+```js
+const app = createApp(MyApp)
+app.use(VueRouter)
+``` 
+
+## Mounting App Instance
+
+After being initialized with `createApp()`, the app instance `app` can be used to mount a Vue root instance with `app.mount(VueInstance, domTarget)`:
+
+```js
+import { createApp } from 'vue'
+import MyApp from './MyApp.vue'
+
+const app = createApp()
+app.mount(MyApp, ‘#app’)
+```
+
+The `mount` method can also accept props to be passed to the root component via a third argument:
+
+```js
+app.mount(MyApp, '#app', {
+  // props to be passed to root component
+})
+```
+
+With all these changes, the component and directive we have at the beginning of the guide will be rewritten into something like this:
+
+``` js
+const app = createApp()
+
+app.component('button-counter', {
+  data: () => ({
+    count: 0
+  }),
+  template: '<button @click="count++">Clicked {{ count }} times.</button>'
+})
+
+app.directive('focus', {
+  inserted: el => el.focus()
+})
+
+// now every Vue instance mounted with app.mount(), along with its 
+// component tree, will have the same “button-counter” component 
+// and “focus” directive without polluting the global environment
+app.mount(MyApp, '#app')
+```
+
+## Provide / Inject
+
+Similar to using the `provide` option in a 2.x root instance, a Vue 3 app instance can also provide dependencies that can be injected by any component inside the app:
+
+```js
+// in the entry
+app.provide({
+  [ThemeSymbol]: theme
+})
+
+// in a child component
+export default {
+  inject: {
+    theme: {
+      from: ThemeSymbol
+    }
+  },
+  template: `<div :style="{ color: theme.textColor }" />`
+}
+```
+
+## Share Configurations Among Apps
+
+One way to share configurations e.g. components or directives among apps is to create a factory function, like this:
+
+```js
+import { createApp } from 'vue'
+import Foo from './Foo.vue'
+import Bar from './Bar.vue'
+
+const createMyApp = () => {
+  const app = createApp()
+  app.directive('focus', /* ... */)
+
+  return app
+}
+
+createApp().mount(Foo, '#foo')
+createApp().mount(Bar, '#bar')
+```
+
+Now the `focus` directive will be available in both Foo and Bar instances and their descendants.

src/guide/migration/treeshaking.md

@@ -0,0 +1,3 @@
+# Global API Treeshaking
+
+<!--Tmp. placeholder for linking purpose-->