Ok, finding that method led me to:
Mouse input does not work on right side of screen in windowed build .
I also raised incident IN-15001.
And the temporary fix;
A fix is on the way. You can work around the issue now by moving the UGUI package from the PackageCache folder into the projects Packages folder. Then replace the file MyProject\Packages\com.unity.ugui\Runtime\UI\Core\MultipleDisplayUtilities.cs with this one Workaround to UGUI input issues https://issuetracker.unity3d.com/issues/broken-mouse-pointer-input-coordinates-in-graphicraycaster-for-certain-window-sizes-in-build-when-run-in-windowed-mode ยท GitHub