as someone with a multi-monitor setup I recently thought to myself: Hey, there are another 1920*1080 pixel I don't currently use and seeing more in Factorio would be really great, especially with mods which add UI elements (bobs inserters, Helmod, personal teleporter, to name a few) which occupy some serious space an the left side. So I switched to windowed mode and expanded the Factorio window over both my screens and set the windows taskbar to automatically hide when not used, leaving me only with the blank bar at the top of the window. Wow you see a lot with that. But what you actually don't see, or at least not in a nice way, are your character and key UI elements like your inventory, since those are centered to the middle of the available space. And that's where one screen ends and the other begins now.
To really make a dual-screen setup work the UI elements and the screen position of your character would need some kind of offset-setting, to adjust them to whichever side you need, or better yet, be movable to a place on the screen to your liking. If there would be an option to remove or at least reduce the window bar at the top Factorio would be really awesome on multiple monitors.
Another thing I noticed while using the windowed mode to spread the window over the screen boarder: The window doesn't seem to save it's position and size. So everytime you start Factorio you have to resize and reposition it.
I get this might be rather niche problems, because not everyone uses more than one monitor and even those who do might prefer to play on only one. Still I think movable UI/character position and proper window behaviour would be a great addition to a great game
