dom class value lost in a special case

[ouzhenkun]

hi~ @developit

in this codepen:
http://codepen.io/ryan_ou/pen/rLOaJa?editors=0010 (5.0.0-beta9)

follow these steps you will see the issue:

1. click Set background button
2. click Hide/Show button 4 times
3. in chrome dev tools - Elements tab you will see:

<div id="panel" class>
  <button>SetBackground true</button>
</div>

it should be:

<div id="panel" class='bg'>
  <button>SetBackground true</button>
</div>

If you change the vnode(id='panel') to be:

<div id='panel' className={hasBg ? 'bg' : 'a_not_exists_class'}>...</div>

the class will be correct.

It seems like that:
If a component.className has been set to null/undefined/'';
a dom's class will lost when preact re-use the component.base

[developit]

Good catch. I believe this line is the suspect:
https://github.com/developit/preact/blob/master/src/vdom/diff.js#L248

[developit]

Ahh - @zhenkunou looks like this is specific to className - the bug doesn't seem to occur when using class.

[developit]

Fix released in 5.0.0-beta10.

[ouzhenkun]

I've updated the demo(5.0.0-beta10), it work now!! Great, thank you.