I did that, but now it wont connect to server with a true display -_- and If I override the resolution, it has the same problem I started with (made a new option.txt)
I kept having the same problem, display kept changing when I would boot up the game, had to keep setting it back and then the game went off screen like before, went back into options and my display is set to 800 600 16. All I did was open and close it and restart the launcher and it worked again. My problem now is that I can't make the client full screen. ( oh and of course the fact that the server is down)
Open option.txt make shore that true display is set to false then change the display settings to yours.
Then do a save as leave the name option.txt and save it to your desktop. Then copy it back to the same folder and overwrite the original option.txt and from then on you can edit it normal.
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you might need to change the permissions on the options file. I had to before it would let me save it.