The focus thing is a bug and should be fixed. Nothing to discuss about it as long as there are no news.
@Luobin I'm using SizeUp on MacOS Mojave (10.14.6) and it works fine with multiple Opera Windows (63.0.3368.107). But yes, using another application should not be the solution.
By the way: The bug is still not fixed as of update 64.0.3417.47