fix(transition-group): fix activeInstance regression

fix vuejs#9151

src/core/instance/lifecycle.js

@@ -21,6 +21,14 @@ import {
 export let activeInstance: any = null
 export let isUpdatingChildComponent: boolean = false
 
+export function setActiveInstance(vm: Component) {
+  const prevActiveInstance = activeInstance
+  activeInstance = vm
+  return () => {
+    activeInstance = prevActiveInstance
+  }
+}
+
 export function initLifecycle (vm: Component) {
   const options = vm.$options
 
@@ -52,8 +60,7 @@ export function lifecycleMixin (Vue: Class<Component>) {
     const vm: Component = this
     const prevEl = vm.$el
     const prevVnode = vm._vnode
-    const prevActiveInstance = activeInstance
-    activeInstance = vm
+    const restoreActiveInstance = setActiveInstance(vm)
     vm._vnode = vnode
     // Vue.prototype.__patch__ is injected in entry points
     // based on the rendering backend used.
@@ -64,7 +71,7 @@ export function lifecycleMixin (Vue: Class<Component>) {
       // updates
       vm.$el = vm.__patch__(prevVnode, vnode)
     }
-    activeInstance = prevActiveInstance
+    restoreActiveInstance()
     // update __vue__ reference
     if (prevEl) {
       prevEl.__vue__ = null

src/platforms/web/runtime/components/transition-group.js

@@ -14,6 +14,7 @@
 import { warn, extend } from 'core/util/index'
 import { addClass, removeClass } from '../class-util'
 import { transitionProps, extractTransitionData } from './transition'
+import { setActiveInstance } from 'core/instance/lifecycle'
 
 import {
   hasTransition,
@@ -36,6 +37,7 @@ export default {
   beforeMount () {
     const update = this._update
     this._update = (vnode, hydrating) => {
+      const restoreActiveInstance = setActiveInstance(this)
       // force removing pass
       this.__patch__(
         this._vnode,
@@ -44,6 +46,7 @@ export default {
         true // removeOnly (!important, avoids unnecessary moves)
       )
       this._vnode = this.kept
+      restoreActiveInstance()
       update.call(this, vnode, hydrating)
     }
   },