Full screen framewire

Hi,
I just installed the PPr24, and I'm having a weird think that I didn't have with the previous versions. When I watch my edit in full screen I can actually see the framewire of the cropped videos. Not the mask-blue-limit-of-the-frame that you can select but a thin white line that marks the limit of the composite video.

This is of course clear because I have different cropped and reduced videos in a same image. As the backgrounds of my images are black, I normally don't see the limits of them. But now I can see it (only) when playing in Full screen.

I found a solution: I created a black video and put it under, in V1. So, now, the composite video is full screen and I can't see the limits of my other videos in v2, v3 etc. But this is so stupid, I have a whole video line blocked that I hadnt' before. It's so Avid and I feel this is strange. When I'm using Avid I think like avid, but when using Premiere I would like to think like premiere, where an empty space means black.

Is there something I'm missing? A button, a preference? Anyone has experienced the same?

thanks.