I just downloaded TDM for Linux.  When I start it, it forces my external display to mirror my laptop display, then displays the game on both monitors, with one somewhat distorted due to it having the wrong aspect ratio.  Even after the quitting the game, it leaves the display setting to display the same image on both screens, which I have to fix manually.   Ideally, I'd prefer it only play on the external display without needing to disable the built in.