Just bought this great U3417W and am going crazy trying to get it to work with my MacBook Pro 2017 with Touch Bar running Apple OS High Sierra. I was assured by at least ten by Dell salesperson that the U3417W would work with my Apple. I am using a USB hub connected to my C port - the one that also charges - and using the HDMI cable to the U3417W. Many issues to even get the connection and even more to get the clamshell option to work so I can close my laptop and use a wireless Apple Magic Mouse and the Logitech wireless keyboard. Have mirror display set on the arrangement. Sometimes it works. Other times the screen on the U3417W shakes back and forth and I disconnect the cable and start over. Help. When I bought and assured it works, nothing said about "sometimes" and ready to send it back.
Apologies for them not knowing our product. The Apple proprietary hardware, operating system, and drivers were not tested/validated by Dell on the U3417W. You can see from the U3417W driver page that Apple was never mentioned. I see Windows Vista/7/8/8.1/10. Also, the U3417W User's Guide only mentions Windows, never Apple. You should post on the Apple Forum and see if they can offer some insight or troubleshooting steps.
I find it very distressing to take the time and energy to order this, be excited and it is a potential tech nightmare. I asked clearly multiple times if it would be compatible, not compatible if I do somersaults. Thanks for your suggestion.