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Dell XPS-15, i7-6700HQ CPU @ 2.60 GHz. 16.0 GB RAM.
x64-based processor, Windows 10 Pro (1803) - 64 bit operating system. (OS Build 17134.706)
Monitor 1 - attached to laptop.
3840x2160. Change size of text, apps, and other items set to 250% (Recommended).
Monitor 2 - secondary monitor, attached via USB3.1.
1920x1080. Change size of text, apps, and other items set to 100% (Recommended).
Displays are "extended".
Note that the above is my "traveling" setup. At home I substitute another UHD monitor for Monitor 2. It has the same resolution of 3840x2160 as the primary monitor. It is connected via the HDMI port on the laptop.
The problem described shows up on either setup.
Note that I tried to change the main monitor to 100%, but the text was so small that I could not read what it was trying to convey. No other program that I use has this problem.