class animation: document what happens with properties that can't be animated

[thomsbg]

Because the animation function is not able to animate some properties (like background-image), an elements appearance may suddenly change at the end of the class animation because that is when the target classes are finally applied.

This feature adds an boolean flag option that allows the target class to be applied at the beginning, so that the non-animatable effects of the changed className are visible during the animation.

[thomsbg]

Code to accomplish the job - ​https://github.com/thomsbg/jquery-ui/commit/96470fd70ca160658ac8ac6ffec3d0e590a07d47

[danheberden]

Great job on your alterations - that should help people stumbling across this bug report understand what's happening when their images suddenly update.

There is, though, some ambiguity in this topic. No matter what you do, the image is going to suddenly change. Fading an image from one to another, as I'm sure you've done, is an entirely different process all together.

While I can totally see why this feature would be helpful for you in your application, I don't think it has a place in jQuery UI. It's a chunk of code that still presents the same core problem: the style can't be animated.

But, because you've posted your fix, others who have an application specific need for this can benefit, so again: thank you.

[thomsbg]

I decided to contribute this piece of code only because the decision of when to update the styles that can't be animated is somewhat arbitrary and not all that well documented. If only there were a commenting feature on the jQuery UI site like there is for jQuery, I would happier leaving this discussion there.

[Scott González]

I've added a note in the new API repo.