Using latest OS X (11.4 El Capitan on a 13" retina Macbook Pro), with 18.104.22.168 Flash Player / Safari Plugin.
At first, I had this specific issue: when bringing a YouTube video into fullscreen, about 10% of the time it would reboot the computer and cause the login screen to appear, even though the Mac's settings are to auto-login and not prompt for the password.
After resintalling OS X, making a new user, AND replacing the logic board, I still have issues. However, to my knowledge, I am not having the full-screen video issue anymore, but now Flash player plugin usage (having a video playing) has caused my mac to totally freeze. I've contacted Apple to send in crash log files, but since all issues have been caused by Adobe Flash Player I wanted to note this here if anyone has any help/support. I have a variety of crash log notes saved in a text file if that's of interest.