Scenario:
Xendesktop 7.9 server connects to physical machines in office. The target workstation is Windows 10 with dual monitors, an nVidia K2200 video card.
The display turns black and unless the power cables to the monitors and switch ports are unplugged, the issue seems to persist.
 

Below are the steps that one can try if they are experiencing an issue with display.
 

1. Try using a single monitor on the physical machine and then check the behavior. 

2. Check the version of Citrix Receiver on the Mac Book. If it is on an older version, try to update it. The latest version of receiver for Mac we have is 12.2. 

3. Try to connect from a Windows client remotely and then check the behavior. 

4. Back up the following registry key: 
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\GraphicsDrivers 

Delete the following registry keys: 
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\GraphicsDrivers\Configuration 
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\GraphicsDrivers\Connectivity 

5. Rename the value in the following registry entry to prevent MMHook from loading entirely: 
HKLM\Software\Citrix\CtxHook\AppInit_dlls\Multiple Monitor Hook\FilePathName 
That is, change the value from c:\Program Files\Citrix\System32\mmhook.dll to c:\Program Files\Citrix\System32\mmhook_test.dll. 
Now test by reconnecting the session. 

6. Allow MMHook to load, but disable it for all applications. 
Create the following DWORD registry entry with a data value of 0, (zero): 
HKLM\Software\Citrix\CtxHook\AppInit_dlls\Multiple Monitor Hook\DefaultHooks 
Now test by reconnecting the session. 

7. Try to logoff the session from Mac and login from the physical machine. And also check the behavior when disconnecting and reconnecting the session.