mouse near start menu not working, control battle

I am trying out Multiplicity and overall it looks like the right thing for what i need.

BUT

one really bad bit has shown up that i need to solve.

so i have 2 computers, one is a Dell laptop the other is a Microsoft Surface Book.

the surface book has the built in screen and an external 4k display

the dell has the lid closed and a 4k display attached.

they are set up like this

DELL4K   SURFACE4K    surfacebook

multiplicty is installed on the dell as the controler.

i can move the mouse over most of the screen without any problems.

but then i am in the area of the surface 4k display  lower left corner   near the windows start icon  then i have a problem.

the mouse goes crazy like it can't work out who is in charge the surface or the dell.  the mouse pointer jumps down to the start menu or over to the dell display.

one other thing that might be part of this is both computers are running windows 10 and have display scaling on so that the 4k display is not making my icons and menus too small.

so possibly something in the math  of where the mouse is and how close it is to the screen edges

gets jumbled ??

but it only happens in that corner near the windows menu icon / start menu.

anyone  seen this ??  know how to fix it ??