Something I observed since a very long time on my Mircosoft Surface Book 2 is now confirmed by Citrix Support:
Citrix HDX session may stop responding and hang when using the two-finger scroll on a touchpad. This was observed in certain machines running Windows 8 or 10 with receiver 4.10 or higher.
There is currently no solution, but the workaround to press a system keys like Alt-tab or windows key to wake up the OS input queue and automatically correct the problem.
Citrix and Microsoft is investigating the issue.
UPDATE 27th of October 2018
Microsoft has finally fixed the issue and published fixes for Windows 10 1709 and 1803:
Windows 10 1803: https://support.microsoft.com/en-us/help/4462933
Marco Klose works as a Technical Consultant and Architect focused on Application & Desktop virtualization as well as application delivery with the Citrix product portfolio. He is specialized in Citrix virtualization, Citrix networking and Microsoft products. He has +10 years experience and holds the latest Citrix certifications and is member of the Citrix Partner Expert Council EMEA (PTEC).