I tried editing the "default" config file to set the lines to "windowed=true" and "borderless_window=true," but they always reset back to false after launching the game.
Then I read the FAQ section that said the in-game resolution has to be the same as the desktop resolution, otherwise it will force disable it, which is what was happening.
I did that, and now the lines aren't changing back, but what is the point of the windowed mode if the desktop and in-game resolutions are the same? I tried changing the in-game resolution to something lower afterward, but it did not revert to windowed mode.
The config file lines are still true though. What did I do wrong?