If I check in through to international area at Suvarnabhumi before midnight on the same day as my visa expires, does that mean I've "left" Thailand and do not have overstay? My flight leaves after midnight...
TLDR : Answer Summary
Yes, if you check in through the international area at Suvarnabhumi Airport before midnight on the day your visa expires, you will be considered to have officially left Thailand, thereby avoiding any overstay penalties. Many users reported that passing through passport control before midnight ensures no overstay issues, and even a one-day overstay usually comes with a waived fine if dealt with properly.