Component `onUnmounted` gets called too easily

[ReneDrie]

Currently if you set an element to display: none, all components within will become unMounted. I don't think that's the desired outcome as we want to toggle a lot usually.

[psimk]

@ReneDrie Could you create an example showcasing the issue on http://codesandbox.io/ ?

[ThaNarie]

Added a test case story, but could not reproduce this.

Most likely there are some additional factors involved, so it would be useful to get more information so we can reproduce it.