I'm sure this question is addressed for other monitors and setups but couldn't find a solution for my setup. I've connected the U2415 via DisplayPort to my Apple MacBook Air. I've connected one mouse and one keyboard to the U2415 as well via USB, and connected the U2415 to Apple MacBook Air via the USB cable.
Apple MacBook Air 13 inch, early 2015
Processor: 1.6 GHz Intel Core i5
Memory 5GB 1600 MHz DDR3
Graphics Intel HD Graphics 6000 1536 MB
MacOS Sierra Version 10.12.2
As you can see in the image below taken directly from the monitor page, the Apple proprietary hardware, operating system, and drivers were not tested/validated by Dell.
Since we at Dell are not trained in the usage of the Apple OS, you should ask the Apple users on their Forum for troubleshooting opinions.
What we would need you to do is connect the monitor to a Windows based computer (not an Apple computer running Windows). We could then troubleshoot with you.