the flash player (game window) goes full screen using F11, or using the zoom in eg. chrome as the game window opens (not easy as the window often opens before you can get to settings/ zoom quick enough to hit full screen)
- however for me at least - all it does is make the game window full screen, the actual playing part within the game window / screen is still 4:3 format with empty black areas to the left and right - ie. does not fill the screen.... If it did, the proportions would be wrong.
Positive is that it doesn't fill the sides up with adverts or annoying promotion videos & pictures - I've sometimes used a blanking template made from an old leather bag (lol) on other things to blank out the side of screen junk (eg on some streams etc).
Noting that flashplayer is going to die in 2020+, maybe earlier, we will see if the game owners (took over the game in 2016 I think) do something to support flash player based pc/laptop usage still.