UserAgent: Mozilla/5.0 (Windows NT 6.3; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/53.0.2785.89 Safari/537.36

Example URL:
https://www.eduonsolutions.com/CmH6kxJYd1zAD2WqqN6Gk/google-example.html

Steps to reproduce the problem:
1. Load the included URL
2. Make the height of the browser window smaller, so just half of the YouTube container is shown.
3. Try the scrollbar, it should scroll the modal window without any issue.
4. Now press the play button to start video playback.
5. Try the scrollbar again, the video will be the only part of the screen which is going to be scrolled. The rest will just break.

What is the expected behavior?
The content should scroll normally, e.g. the whole modal should scroll OK even if the video is being played.

What went wrong?
This first appeared on v53 Canary, sadly I missed the concrete version when it was introduced. From that point, I waited if someone would spot this, as creating a small example seemed too difficult. Now it is in Stable and I had to spend time on it. Other browsers show this without any problem, even the newest Opera.

Does it occur on multiple sites: N/A

Is it a problem with a plugin? No 

Did this work before? Yes v52 Stable, first versions of v53 Canary

Does this work in other browsers? Yes 

Chrome version: 53.0.2785.89  Channel: stable
OS Version: 6.3
Flash Version: Shockwave Flash 22.0 r0

It might be that we are doing something against CSS standards. If that is so, feel free to tell me.

Also, this is not just connected to video elements being played. It also happens when there is a CSS animation running over some regular HTML element - after the animation finishes, the page starts to behave correctly immediately.