The new display settings dialog in 4.11.3 is pretty awesome. However, it misses just one little feature I use all the time -- inter-screen spacing. Specifically, my current setup looks like this: left screen: 1280x1024 at 0x0 right screen: 1600x1200 at 1356x0 so there's some dead space between the two screens. It is helpful for two reasons: 1. if I accidentally move the mouse near the edge of one screen, it is less likely that I accidentally click something near the edge of the other, 2. shadows of maximized windows don't appear on the other screen :). It would be great if xfce4-display-settings could support this kind of configurations. However, I should note that some display drivers require output positions to be aligned at some boundary. I don't know the exact numbers but I think multiple of 16 should be relatively safe.
Making it possible in the dialog to set dead space would break the snapping feature of aligning two displays next to each other, which is the common usecase and expected behavior. Also, users might "lose" stuff if they mistakenly do this setup and the space between two screens is too big. So yeah, I've discussed this briefly with Sean and we don't want to have that kind of functionality in the UI of the display settings. You can still easily do your setup for yourself though and I suppose all others who want this setup will be able to figure it out too.