Well I reset the device to clear a few things (Settings > Reset > Restore settings) and now it appears mcompositor.conf is missing from /home/user/.config/mcompositor. The folder is just empty (new-mcompositor.conf is not there either, but I had mcompositor.conf before).

I seem to remember there were quite a lot of settings in this file. Isn't it bad that they are all gone?