It’s a long-in-the-tooth holdover from the Windows MDI (Multi Document Interface) and won’t go away until they roll-their-own dockable UI – ala Studio One, Sonar, et al.
It’s non-trivial. It will be a big deal when they do and will be hugely marketed.
I feel its pain more than most, because I “synergy” (mouse keyboard share) to a Mac Pro running other music-related software.
Every time I do that, move the mouse to the other screen, all my Metering plugins go away – just when I need them the most. It’s maddening.
I have to flip the mouse back and forth to follow my gaze.
You, on only one platform, can avoid it by stretching the background window across the screens – something I can’t do in my PC / Mac configuration, but is what I do for Cubase itself running on my two main screens on my PC.