I am using TotalSpaces 2 under Mac OS 10.13.2 & have had this issue for ages - in using a dual monitor view for Capture One Pro I have the Browser on one screen and the Viewer on the second screen. Thru preferences I can easily lock the Browser to whatever screen I want. But even when I specify which screen the Viewer should appear on, It wanders all over the second monitor from screen to screen to screen. It may have something to do with Capture One itself because whenever I use exposé to see all my screens, Capture One’s Vewer becomes invisible (only under exposé). The Browser IS visible under éxpose. There seems to be no way to lock down the Viewer on monitor 2 to a specific screen.