The desktop is incorrectly displayed in the Desktop Viewer. The display or screen size is smaller than expected or corrupt:

NOTE (22nd December 2015): Please be advised if hotfix ICAXS760WX64042 is applied the below settings might revert back to defaults. We are actively investigating this issue under a specific case.

Complete the following steps to edit the registry key on VDA:

Caution! Refer to the Disclaimer at the end of this article before using Registry Editor. 

1. Open registry and navigate to:
   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\vbdenum

2. Change or add the following registry values:
   "Start"=” 0x00000001”: REG_DWORD
   "Group"="EMS": REG_SZ
   "MaxVideoMemoryBytes"=”0x08000000”: REG_DWORD

3. Navigate to:
   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\vd3d

4. Add the following registry values:
   "MaxVideo​MemoryBytes"=”0x08000000”: REG_DWORD
   
   Note: The MaxVideoMemoryBytes key(s) should only be used in conjunction with the "Citrix Systems - WDDM Display Driver".  If the "Citrix Display Only Adapter" is in use then these keys should not be set as memory is allocated dynamically.

Problem Cause

In the VDA, the Citrix WDDM driver allocates 128MB memory by default for video memory buffer at boot time.

Physical memory fragmentation would occur if other installed drivers allocate memory at boot time earlier. In this case, the Citrix WDDM driver is not able to allocate sufficient RAM for video memory buffer and causes the issue.

CTX200257 - Screen Issues Connecting to 4K Resolution Monitors in DCR Mode