With the xxx15 monitors, once you switch away from a Windows source, you can't switch back from a Windows source because it is no longer connected to the monitor. In your case, once the monitor is connected to a Mac source, your best bet is to use the monitor OSD shortcuts to configure a button to switch back to the Windows source.
Hello,  I'm running into an issue here where I'm only able to switch from DP to HDMI (work), but not back from HDMI to DP (main). Using the OSD to change input sourcing on a day to day basis becomes quite annoying. I'm unable to download DDM on my MacBook as it's not compatible with macOS for the U3415W at least.  When attempting to switch from HDMI to DP (main), I know my PC doesn't detect the U3415W anymore which is why this issue is probably apparent. I'm probably missing something that's obvious, but looking for the answer if available! This is after following: U2415, DDM, changing source using keyboard main (doesn't work) [IMAGE] work (works) [IMAGE]  
U3415W, DDM, cannot switch from HDMI to DP