This pull request is also fixed #9364.

The test-e2e failed, I thought it was a problem with CircleCI, so I ignored it. Today I found that the test case was not updated. But also found some other hidden dangers, this PR does not seem to be compatible with the previous version, suppose there is a component of my-transition, his template is <transition><div>xxx</div></transition>, if used like this:

<my-transition v-if="showing"></my-transition>

When showing is changed from false to true, the previous version will have a transition effect, but the new PR will result in having to specify an appear attribute for the transition to have a transition effect.

I disagree with that this change is not breaking things.
Yeah, the app builds etc.

But enter animations aren't called without applying appear inside a transition wrapping inside a component. Sure, just add appear – problem now is that the component's enter animation is triggered on first load also, which is not what I want.

See this issue: #10227

@yyx990803 @shasharoman

Just found an example on the web while trying to understand how to use transition as the root element of my component.
I was wondering why the codepen example worked, but not in my project !
The example use Vue 2.6.4 which was before this patch was merged.

Vue 2.6.4: https://codepen.io/tahazsh/pen/mvKjbq
Vue 2.6.9+: https://codepen.io/mrleblanc/pen/eYgyZqR

I think the documentation should be improved on how to handle transition when they are on root element.
Currently if you do not specify appear, the leave transition work, but not the enter transition.
All example on the doc use regular HTML and not components which mean we have to guess this behavior.
Also it seems odd that we need appear when <v-transition> is at the root, it think there should be an exception for this behavior.

If you agree, I can open a new issue and reference this PR.