Desktop moved from primary to secondary monitor (by a ghost??)

I have a Gigabyte P27 laptop with a Dell U2913WM monitor attached via VGA connector and a Samsung TV connected via HDMI.
I use extended desktop and the Dell is set as Main Display. The built in monitor is disabled.
The configuration is, from left to right, Dell (3), TV (2), Built in (1).
I been running this way for about a year with no issues until two days ago when all of a sudden after boot up the desktop had disappeared from the Dell and appeared on the Samsung. But the taskbar stayed on the Dell.
I dragged and dropped the stray icons back to the Dell and all was fine until the next restart when they moved back to the TV.
I have since discovered that any screen refresh causes the same phenomenon.
I've hunted high and low but I can find nothing that would explain this or suggest how to fix it.
Any help or info would be greatly appreciated.

bro67 said:

That is because the laptop will always be Display 1. No way to change it.

?????

I'm sorry, but how is that relevant?
The Laptop's built in monitor is not the issue.
The issue is that all of a sudden Windows has apparently decided to dynamically swap things between the two external monitors.

Installed DisplayFusion.
That's fixed it.
However still can't explain what caused the problem.