When I make a selection in photoshop 2018 using the marquee tool the edges are not visible. I have to click in the marquee box in the toolbox and then they become visible. This is across a number of machines using Sierra on Macs.
This issue never manifested with 2017. It did however with a previous version, the original CC if I remember rightly until an update fixed it. I'll just keep all machines on 2017 and periodically check to see if the issue has been fixed in future updates.