Defocusing freezes the game

[XLambda]

This happens pretty often on my Ubuntu 15.10 system. Whenever I defocus ToME (by pressing the meta key) the game may freeze and require me to kill not just the game process, but the entire session. This is pretty nasty because the meta key is located directly below the level change and autoexplore keys on my keyboard, so it's extremely easy to press it accidentally when playing.