Citrix Desktop Viewer is unable to reach the correct resolution of a 4k monitor. Max resolution by the monitor is 3840 x 2160 but when launched Desktop Viewer is able to reach only 2560 x 1440.

Changing the High DPI settings to use the native resolution (Advanced > High DPI, Select "No, use native resolution") does not resolve the issue.

Following CTX230017 shows "Disable display scaling on high DPI settings" checked and greyed out for all users.

1. Remove the entry for cdviewer.exe under: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\AppCompatFlags\Layers it will be called something similar to c:\\program files(x86)\\Citrix\\ICA Client\CDViewer.exe and have the value ~HIGHDPIAWARE
2. Change High DPI scaling (under: Advanced-> High DPI,) to "No, use native resolution"

Problem Cause

GDI processing was disabled for CDviewer.exe by windows, so whatever setting in Advanced > High DPI was ignored by the OS. Hence we were unable to force receiver to send the full resolution of the 4K monitor because the operating system was providing Receiver with the augmented resolution so scaling monitor size to get equiv DPI size. (3840 x 2160 is 150% of 2560)

Roberto Moreno's blog post: https://www.citrix.com/blogs/2018/01/09/making-sense-of-high-resolution-displays-and-dpi-with-citrix-graphics-remoting/