On Win10Pro with 3 monitors. Successfully running ET on one monitor full screen, 1920x1080. But, the mouse doesn't stay locked to ET when running/playing. Mouse can be moved off dedicated monitor playing ET to another monitor without issue. Is there a way to lock the mouse to the ET session?

Not an ET only thing, found many game players have the same issue across many games. Some getting the mouse to stick with monitor tools type software. Thanks for reading

Perhaps
https://steamcommunity.com/sharedfiles/filedetails/?id=2800452649

thanks >M@tEo$<, already been through that and talked with discord guys, unfortunately either unique for me, but not many others have a solution

Have you tried too:
Settings > System > Display
And see if the main/ dedicated monitor is selected?
Then select the monitor you want and check the box for "Make this my Main monitor"
(Bottom, under Multiple displays)
Hope this helps.