Yes multi monitor setups have issues, like currently at my home computer factorio is running but I cannot connect to it as it's located on my secondary monitor, as factorio has an internal setting that opens the window to that monitor, initially it was o, however due to network error and packet loss the initial connection was severed and I can no longer connect to he game, if I open another game I can hear factorio main menu sounds under the game I cannot close the game via steam client nor can I invoke the popup that would allow the game to be turned off.
I know this is an old issue, but still relevant.
also games that won't comply with steams graphical settings render on the original resolution and you are stuck with a game that renders full screen and a binding box where your mouse is stuck, unable to change the resolution. and how this is a multi monitor issue, honestly idk but it is.
Nothing extracted yet.
Your system information
Steam client version (build number or date):

Distribution (e.g. Ubuntu): NixOS 21.05
Opted into Steam client beta?: No
Have you checked for system updates?: Yes
Please describe your issue in as much detail as possible: