@Proton2: 4k monitors probably won't be run at 1:1. The text would be too small to read. I think they'll most likely be run at 2:1 to get a nice DPI boost.
I agree with your suggestion of showing running desktop apps in the left side menu.
8.1 will prompt the user to choose a pane if you open something when already split. It kind of hovers in the middle.