How to play Fly Casual in windowed mode

You need to pass additional parameters to it's executable file in commandline (or to create shortcut with these parameters). Examples of commands for windowed mode with resolution 1280x720:

Windows: FlyCasual.exe -screen-fullscreen 0 -screen-height 720 -screen-width 1280

MacOS: open -a /Applications/FlyCasual.app --args -screen-fullscreen 0 -screen-height 720 -screen-width 1280