Surface Pro 3 external display and Citrix XenApp: https://community.spiceworks.com/topic/663199-surface-pro-3-external-display-and-citrix-xenapp
https://support.microsoft.com/en-in/help/3025083/windows-scaling-issues-for-high-dpi-devices
Versions of Citrix Receiver for Windows 4.10 and higher are now "DPI scaling aware", and provide improved support for handling higher DPI resolution in a session. To learn more about high DPI scaling, please visit the following links -