Flickering black background on the extension panel

Hello!

I love Adobe Animate, I've been using it for a long time. I have a YouTube channel associated with this program. There are already 9000+ subscribers and 4500 views every month. A little over a month ago, I took a course in JavaScript and as practice, I decided to write an extension for my favorite program.

I created the framework for the extension in "Creative Cloud Extension Builder for Brackets", then adapted the CSS to the Animate interface and built the panel into a convenient place.

 
The problem is that when I switch between panels - a black background flickers! No such thing with the native panels. Immediately thought on my CSS and tried loading the standard CSS "Extension Builder for Brackets" - same thing, black frame. That is, the problem is not in Animate and not in my CSS, but where to look for the problem I do not know.

Perhaps someone has faced with similar or at least have a suggestion what could be the problem?