Stop the mouse from moving between PCs when the mouse button is down

[NeoTechni]

If we're dragging something, it should not let us switch monitors since we can't drag anything between monitors anyway

That way we can dock windows to sides of monitors again.

[Psi-nz]

Also this would fix the issue where you are using a vertical scrollbar at the side of the screen and have to be very careful not to ever move the mouse a few extra pixels over and hit the screen edge or your scrolling is terminated and it jumps position back to before you started to scroll