I’ve also been “dealing” with this issue. I was hopeful that 4.8 preview would address the viewport selection problem, but no luck. I’ve been bouncing between the use of ‘widget scaling’ and getting real close to my new Samsung 28" 4K monitor. It’s a lovely display, but Windows, and most apps don’t react very well to such high DPI… Meanwhile, when I hook up my MacBook Pro, it just works. (slight rant)
Epic, any plans on addressing what seems to be just a few more oddities with high-DPI support? Don’t all your devs have 4K displays yet? 
Disclaimer: No complaints, formally… wonderful toolset, great company…