Flash player not using entire screen (in full screen) or app/video's designated entire area in regular non-full-screen mode

What it says above.  I am using Windows 10 and Flash seems to be especially buggy/wonky under the new OS.  It isn't using the entire surface area that's designated for the app/video or whatever flash thing I am using.  Could this be because I'm using a 4k display and my rez is set to 4k?  Screen captures attached that show this issue with an example game (Jigsaw World on Facebook). This unuse of the entire display area also happens with other websites, including MLB.TV and a few other places.  Any idea what could be causing this and how do I fix it?