If you only write “FullscreenMode=2” (Window mode) and nothing else in the ini file, it will ignore it. I think you need some of the tags. “[/Script/Engine.GameUserSettings]”, but I have not checked that out myself. You can start the game, press ok in menu, and GameUserSettings will be generated with settings.
[ScalabilityGroups]
sg.ResolutionQuality=100.000000
sg.ViewDistanceQuality=3
sg.AntiAliasingQuality=3
sg.ShadowQuality=3
sg.PostProcessQuality=3
sg.TextureQuality=3
sg.EffectsQuality=3
sg.FoliageQuality=3
[/Script/Engine.GameUserSettings]
bUseVSync=False
ResolutionSizeX=1920
ResolutionSizeY=1080
LastUserConfirmedResolutionSizeX=1920
LastUserConfirmedResolutionSizeY=1080
WindowPosX=-1
WindowPosY=-1
bUseDesktopResolutionForFullscreen=False
FullscreenMode=2
LastConfirmedFullscreenMode=1
PreferredFullscreenMode=1
Version=5
AudioQualityLevel=0
FrameRateLimit=0.000000
DesiredScreenWidth=1920
DesiredScreenHeight=1080
LastRecommendedScreenWidth=0.000000
LastRecommendedScreenHeight=0.000000
I just got an idea to make a workaround. If ini file its set up so game always starts up in windowmode (Where it works with titlebar), I can then apply the saved screenmode after game starts up from my save file.
Then every time settings are saved, it is set to window mode in the ini file, but the correct screen mode in stored my settings save file. You lose access to change fullscreen in ini file. But I think that is an acceptable loss for now. I will try it out later.