Akito do you have a link where we can experience it?
It doesn't happen when I open the reply dropdown on this very discussion while your video is playing.
Is it a posted post, or the preview of a post currently being written? Does it happen only with YouTube, only with the first post, ...?
You can try to replicate here in the "Test posting" tag. If it doesn't happen on this forum, then it's probably caused by another of your extensions. I believe a few extensions use HTML manipulation on the post so they might be responsible for this behavior.
