Hi Andries van der Gugten,
Thank you for reaching out to Microsoft Community. We are sorry to hear about the issue you were having on your Surface Book. We also appreciate the time and effort you spent in trying to troubleshoot the issue.
We would like to ask for few more details to further isolate the issue:
Does the mouse pointer moves to the right corner while you are using it or it only moved when it is idled?
How about if you use Extend display but set 1 of the external monitor as the main display, does the issue occurs also? on the Surface screen?
May we ask if you were able to try using another Surface on this set up? Does the issue still happened?
For now, please try to clear the display cache of the device: Refer to this page on how to Clear your display cache (Solution 5).
If this answer your question, we highly encourage you to inform our Community by clicking on Yes or marking my post as an answer. This will help other members who may have the same issue as you. Otherwise, don't hesitate to update us so we can further assist you.
Kind Regards,
Marrion Y.