doesnt work
ticking fullscreen is still borderless windowed not full screen
even changing res is just changing the rendered res not actually changing the display settings

You may just try to turn of your secondary display before you start the game and then re-enable it when you're done. I haven't played this on dual monitors so I don't know if there is any benefits or if its possible, just a suggestion.

Just don't smack your mouse all the way to the other side lol; there's 3rd party applications and I think a window application that will allow you to lock your cursor on said monitor.

Copy and paste this to your launch options for Valheim.

-window-mode exclusive

Should disable Borderless Fullscreen. No idea why there isn't options for true fullscreen =\

-window-mode exclusive

does not work
the game will not capture the mouse

also looking for a way to make the game minimize when focus is lost

Just set your resolution in the settings panel to something different and turn off ''fullscreen'.

I use a 16/10 monitor which is 1920/1200 pixels. Setting res to 1920/1080 without fullscreen works fine and loss of vertical res is insignificant.