MacOS – OSX / Chrome keeps jumping back to video when in fullscreen

You could assign a shortcut to a script like this:

tell application "Google Chrome"
    repeat with t in tabs of windows
        tell t
            if URL starts with "http://www.youtube.com" then
                execute javascript "player = document.querySelectorAll('#watch7-player embed')[0]
if (player) {
    player.getPlayerState() == 1 ? player.pauseVideo() : player.playVideo()
} else { // if youtube.com/html5 is enabled
    document.querySelectorAll('.html5-player-chrome > button:first-child')[0].click()
}"
                exit repeat
            end if
        end tell
    end repeat
end tell

For Safari, replace execute javascript with do JavaScript.

Here's my workaround: right click on the 1/4-screen video and, if you're using Flash 15.0.0.xxx, you should see a Stretching is <type> item in the options list. Click on that and the video will fill the screen. Keep repeating until you get back to the 'Stretching is none' setting, unless you prefer something else.

I have the same issue when using firstrowsports.eu on my 2014 MBP, multiple different browsers, no plugins, no additional screen, hardware acceleration on and off, Mavericks and now Yosemite.