I'm using Flash version 126.96.36.199 (latest) and have a dual monitor setup on Windows 7 64bit.
Everything worked fine but lately, without upgrading anything, just all of a sudden, the full screen option in flash doesn't want to work as it used to. When I full screen a video on my right monitor it will go full mode on the left monitor, which is very annoying because the left monitor is bigger than the right one. Before it would open the video correctly in what monitor the browser is opened.
I tried things like IgnoFlash , I also tried FlashPatch (by Amiya Patanaik). Neither of them solved the problem.
Below is a screenshot of how the problem looks like. Note that when I have my browser open on my left monitor, it does open full screen mode correctly on the left monitor.