Gray Bar in fullscreen mode multidisplay

There's a solution that I've found which should solve this issue with any application that you encounter this with.

1. Download the free application Nocturne
2. Install, launch, and click the icon in the menubar
3. Select Preferences > then check Dim when inactive
4. Adjust the slider all the way down when desired
5. Enjoy! :)

I've been using this application for ages, and without a doubt it's one of my favorites.

Yes, I have experience with this. Mostly in non-Apple browsers (Chrome). The comments help with stand-alone video players but if you're using a browser in combination with a plugin like Flash or Silverlight, it seems that the browser doesn't take multi-menu bars into account and thus a menu bar size grey/black bar is left at the top.

If you on the other hand use Safari with these same plugins, you'll get "full" fullscreen.

Currently I only watch fullscreen Netflix via Safari.

I believe it's up to the developers of the specific application to fix this issue. Until then, you'll have to use specific applications, or, as the comments suggest, change the fullscreen mode in VLC.