test: add test case for vuejs#6687 + fix lingering comment node

src/platforms/web/runtime/modules/transition.js

@@ -181,12 +181,12 @@ export function leave (vnode: VNodeWithData, rm: Function) {
   }
 
   const data = resolveTransition(vnode.data.transition)
-  if (isUndef(data)) {
+  if (isUndef(data) || el.nodeType !== 1) {
     return rm()
   }
 
   /* istanbul ignore if */
-  if (isDef(el._leaveCb) || el.nodeType !== 1) {
+  if (isDef(el._leaveCb)) {
     return
   }
 

test/unit/features/transition/transition.spec.js

@@ -1081,5 +1081,47 @@ if (!isIE9) {
         }).then(done)
       })
     })
+
+    // #6687
+    it('transition on child components with empty root node', done => {
+      const vm = new Vue({
+        template: `
+          <div>
+            <transition mode="out-in">
+              <component class="test" :is="view"></component>
+            </transition>
+          </div>
+        `,
+        data: { view: 'one' },
+        components: {
+          'one': {
+            template: '<div v-if="false">one</div>'
+          },
+          'two': {
+            template: '<div>two</div>'
+          }
+        }
+      }).$mount(el)
+
+      // should not apply transition on initial render by default
+      expect(vm.$el.innerHTML).toBe('<!---->')
+      vm.view = 'two'
+      waitForUpdate(() => {
+        expect(vm.$el.innerHTML).toBe('<div class="test v-enter v-enter-active">two</div>')
+      }).thenWaitFor(nextFrame).then(() => {
+        expect(vm.$el.children[0].className).toBe('test v-enter-active v-enter-to')
+      }).thenWaitFor(duration + buffer).then(() => {
+        expect(vm.$el.children[0].className).toBe('test')
+        vm.view = 'one'
+      }).then(() => {
+        // incoming comment node is appended instantly because it doesn't have
+        // data and therefore doesn't go through the transition module.
+        expect(vm.$el.innerHTML).toBe('<div class="test v-leave v-leave-active">two</div><!---->')
+      }).thenWaitFor(nextFrame).then(() => {
+        expect(vm.$el.children[0].className).toBe('test v-leave-active v-leave-to')
+      }).thenWaitFor(duration + buffer).then(() => {
+        expect(vm.$el.innerHTML).toBe('<!---->')
+      }).then(done)
+    })
   })
 }