Fix instance reuse bug in out-in transition #1874

[PeachScript]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

This is a PR for #1874 , cc: @LinusBorg

[posva]

Thanks, tested and looks good. I want to see if it is worth to add a test before merging

[PeachScript]

@posva my pleasure! This is exactly what I want to ask, should I add unit test for this bug?

[posva]

I think adding a unit test for this would be a bit too complicated at the moment 🙁
So, don't worry for the moment, this looks fine. I'll have to test manually on IE, if you can check that (IE 9-11) I'd be grateful but don't worry if you can, I'll eventually do it anyways 🙂

[PeachScript]

@posva I'd be glad to help :P

I've tested IE9 ~ 11 in the virtual machines, the transition feature looks well and this bug did not happened, beforeRouteEnter and mounted can get the correct instance every time.

Screenshots:

[PeachScript]

And other examples also work well, except the discrete-components example, it will throw error because the NodeList has not forEach method in IE:

// examples/discrete-components/app.js L33
document.querySelectorAll('.app').forEach((node) => {
  new BaseVue({
    el: node
  })
})

[PeachScript]

Hello @posva , any update on this?