external monitor mirroring laptop monitor in Linux

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.

I've actually got the same situation with dual-monitors on my Linux desktop. I was recently reminded that this happens out-of-the-box by wiping settings for the 2.08 beta (where it does still happen). The workaround I've always used is to run it windowed at the full resolution of my monitor. Then (in KDE at least, I'm sure others have similar settings), apply window- or application-specific properties to select a monitor, remove the window border, and disable compositing. This also has t