Windows seems to assign numbers to displays according to some hidden rules (perhaps something hardware or driver-specific). Using multiple displays through RDP seems to work well (ie. without messing up all your window positions) only if your display layout, size, and display numbers all match correctly. Display layout is easy to alter, but display numbers are not. I have two systems with the same display sizes and layouts, but the display numbers (as seen in the Display Settings dialog) are not the same and I can't seem to get them to be the same. One of the displays is 4K, and as a result, I can't just rearrange which displays are connected to which ports (can't do 4K through DVI!). Is there some way to tell Windows to renumber the displays, perhaps through a registry setting so that I can get multi-display RDP to work between these two systems?
Note that this isn't a question about whether or not RDP will "work" or whether or not the windows can be arranged a certain way. RDP has for many years "worked" with multiple displays. The issue is that when using RDP in this configuration, unless your display numbers, layout, and resolutions match exactly, when you log into your remote system, most of the windows will rearrange themselves to a new seemingly random display. If you have a lot of windows (as I do), this can take several minutes to correct, making the use of RDP a much less appealing solution, because when you return to the desktop you've RDPed into, of course it will perform the reverse rearrangement of windows and screw everything up again. I'm not too keen on resetting the positions of all my windows every time I log in, so this is really annoying behavior.
For example, my first system has displays 1, 2, and 3, with 1 being 4K and the other two being 1080x1920, with the two smaller displays being physically positioned to either side of the larger 4K display. The second system has displays 1, 2, and 3, with 3 being 4K and the other two being 1080x1920, set up with the same physical layout. Every time I RDP from one to the other, the windows on display 1 stay on display 1, the windows on display 2 stay on display 2, and the windows on display 3 stay on display 3, even though only display 2 has a matching resolution, and none of their logical positions match. The "main" display is the 4K one in both cases.
This sad state of affairs can only be explained by the developers at Microsoft not using their product at all, or using completely uniform hardware for their testing, which is totally unrealistic.