I have been using an external Samsung 24" high resolution monitor when I'm using the XPS15z at home (at my desk) with no issues.
Today I decided to go for the Windows 10 Pro upgrade. It took a few attempts (like demanding I uninstall the Cyprus driver, and a couple other glitches related to Avast and other security software).
After completion of Windows 10 installation, I could not use the external monitor (Windows 10 says I do not have a driver to support a 2nd monitor, and no monitor was detected).
Another bigger issue is the resolution. My Windows 7 settings have always been 1920x1200.
Now I can't go above 1020. This is really annoying.
My laptop also has an NVIDIA GeForce GT 525M, 2GB video card, which has far more than this miserable 1020 resolution.
I hope Dell will assist in getting these drivers updated soon.
Thanks.