Scenario:  You have access to your desktop computer through Citrix RemotePC.  You start a Citrix RemotePC session at home.  You wish to keep the session open so that you can continue working at the office, so you disconnect, leaving the session active on your desktop computer at the office.  When you get to the office and press Ctrl-Alt-Del to log on, you might find the display layout to be out of order.   The monitor that is in the left pane moves to the right pane, or vice versa.

• This occurs when re-connecting to an existing Citrix session on the physical computer.
• This does not occur when the user creates a new session locally on his/her physical computer. 
• This occurs only with Windows 10 configured as a Citrix RemotePC VDA.

Caution! Using Registry Editor incorrectly can cause serious problems that might require you to reinstall your operating system. Citrix cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk. Be sure to back up the registry before you edit it.

On the desktop that has the VDA installed:

Created a DWord key named “UseSDCForLocalModes" under “HKEY_LOCAL_MACHINE\SOFTWARE\Citrix\Graphics” and set the value to "1"

If SASNotifications are enabled as described in RemotePC documentation under Session Management, this registry change will have no effect.   
https://docs.citrix.com/en-us/xenapp-and-xendesktop/7-15-ltsr/install-configure/remote-pc-access.html

Disabling SASNotifications (either removing or changing the value to 0), will allow UseSDCForLocalModes to take effect.  

The issue can be avoided by logging off the Citrix session remotely (before travelling to the office).

Sometimes the issue can be cured by swapping the order that the monitors are connected. 

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Problem Cause

This issue affects Windows 10 RemotePC only.  It came about when Microsoft deprecated a display config API in Windows 10.  UseSDCForLocalModes instructs Citrix VDA to use the modern API.
The modern Microsoft grahics API is not compatible with  SAS Notifications, and so cannot be used together with SAS Notifications.