If you use a specialty mouse from a Windows PC, those functions only work if the manufacturer has Apple-compatible drivers for its unique operations. I plugged in a USB hub with a 2ft wire and move the keyboard and mouse to the hub and it's worked flawless for both computers since then. Although a Bluetooth mouse can pair with your Mac relatively easily, the USB version has a few extra steps to follow. I finally found a reference to the issue in the U3415W User's Guide that indicated that there could be contention or interference issues with devices connected to the USB ports on the back of the monitor and recommended that the connections be moved 'away'. Any heavy activity on the MacBook Pro and the mouse would come to a standstill. Rebooting would help for about 5 minutes. I tried lots of things moving to the Thunderbolt DisplayPort (caused wireless network dropping issues). The MacBook Pro, however, developed a very stuttery mouse and latent keyboard activity. The Debian Mac Mini runs flawlessly with the U3419W KVM. Both computers are connected using the HDMI and USB 3.0 cables and a wireless mouse and keyboard each with their own USB connection fobs. Disconnect the USB drive from your Mac mini.
I have my U3419W monitor acting as a KVM for a Mac Mini running Debian Linux and a MacBook Pro laptop. Position the mouse pointer over Devices in the sidebar and click the word Show.
Just in case someone has a similar problem. This isn't a post with a problem, it's a post with a solution.
