===== SOLUTION =====
See https://answers.microsoft.com/en-us/win ... 6683457680
I'm looking at 4 screenshots. Apparently I can't add the screenshots to this post, so I'll describe them. I hope someone can make some suggestions.
But first, this tribute: I can only imagine how difficult the task, and what a triumph ResidualVM really is. I heartily applaud the ResidualVM team for their incredible effort and success!
OS: Win-10 v.1803 with 'Settings' > 'Display' > 'Scale and layout' set to 200%, 'Resolution' set to 3840x2160 & 'Advanced scaling settings' set to OFF.
Screenshots (all are with widescreen_mod=true)
#1, fullscreen=false (1280x960 in 1284x1014 window)
#2, fullscreen=true (1920x1080 black matted to 3840x2160)
#3, fullscreen=true fullscreen_res=1920x1080 (1920x1080 black matted to 3840x2160)
#4, fullscreen=true fullscreen_res=3840x2160 (7680x4320 cropped to 3840x2160)
#1 is as expected. I guess that 1280x960 is some sort of native resolution. (Note that screenshots from the in-progress game are 1280x720 black matted to 1280x960. 1284x1014 is the window dimensions, including frame & titlebar.)
#2 is as expected (matted out to 3840x2160) though I don't know why the unmatted image is 1920x1080.
#3 is entirely as expected.
#4 is totally wierd. Instead of 3840x2160, the game appears as though 7680x4320 (which is then cropped to 3840x2160 so that only the south-west quarter of the game is visible). How much of this is caused by Windows and how much is the fault of ResidualVM is unknown.
Note: Because the game supports OpenGL, I'm able to utilize my laptop's NVIDIA GTX 980M to run it (instead of using the Intel HD Graphics 530 GPU that my laptop also has). Unfortunately, the NVIDIA Control Panel doesn't give me any control over screen resolution, either for desktop or for individual games. (The Intel GPU doesn't have any such control, either.) I find the inablility to scale individual applications surprising, and it's a subject I'm going to take up with Nvidia.
Thanks for your attention.