Ongoing transitions break when browser tab on background

[azamat-sharapov]

1. Open this simple slider on Vue.
2. Wait for slider to start and open browser console on that page.
3. Switch to another browser tab, wait a minute or so and come back to fiddle and look at console, you should see error. (you can open console as a separate window, so that you can look at it and wait until error popus up).

Error: Uncaught TypeError: Cannot read property '__v_trans' of null

[yyx990803]

This is caused by transitionend events not firing until the page regains focus, and only happens for out-in mode. Fixed in dev branch now.