I've been playing fine on my Macbook Pro's attached 16:9 screen for more than a year and never had this issue with the launcher.
After clicking 'Play' in the launcher, the application launches on my main screen (native MBP screen) and then immediately and automatically goes to fullscreen. I cannot find a way to un-maximize it so I can move the application to my 16:9 monitor attached via HDMI. I have always been able to do this until 1.12.2 as it did never launched into fullscreen mode automatically before.
Hope this can be fixed easily.