Problem with full screen on second display

hey guys,

I got a problem.
If I watch a stream on my second display and mute the sound (only in the stream), and click at a window on my macbook display (dosen´t matter which kind of window) the fullscreen mode from my second screen goes off and its into the browser. If i start the volume again, it stays into the fullscreen if I want to work on my macbook display (or checking emails, stuff like that).

How to fix that? If I just want to see a littel clip on youtube I just mute the sound on my stream, but it is annoying that it goes out of fullscreen.

 

I read in an article, that this is normal and I cant do anything against it.
Is this right?