Flash Player does not stay full screen on a 4K display

Adobe Flash will not stay on full screen on a 4K monitor (Pay attention at the 9 second mark of this 240 fps video ... see below)

I have noticed this through several iterations of the Flash Player. I have the latest version of flash player (checked today) and it still happens - when you click the full-screen button, it switches to full screen and stay there for a fraction of a second and then quickly shrinks to fill 2/3 of the screen.

It does not exhibit this issue when the screen resolution is dropped to 1080p. I was able to duplicate  this on several browsers on my Mac - Safari, Chrome and Firefox. It does it on Mac OS X Yosemite and now on El Capitan. It also does it on Windows 7.0 laptop  that is capable of 3840x2160 resolution on  4K monitor.

The video is take at 240fps using an iPhone to show the transition from Full Screen to 2/3rd screen.

Adobe Flash will not stay on full screen on a 4K monitor (240 fps) - YouTube

Here is a good sample video you can use to see if you can duplicate the problem - obviously, to duplicate the problem play it on a 4K screen.

http://video.foxnews.com/v/4604517699001/blake-shelton-and-gwen-stefani-get-flirty-on-the-voice/?playlist_id=trending#sp=show-clips