Launch the game and set all the options to anything you desire, then quit it and change the 16:9 resolution to your desktop resolution in the registry at
HKEY_CURRENT_USER\11BitStudios\Moonlighter\
The values should be decimal.
This should result in the city level and main menu being hor+, HUD spanned. It seems the dungeon levels and the shop simply don't have the content to display at wider resolutions. I haven't really played the game to tell you if there are other types of levels.
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moon 21.png [ 361.26 KiB | Viewed 3449 times ]
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moon 16.png [ 295.07 KiB | Viewed 3457 times ]