ResidualVM, the wrapper that adapts Myst 3, apparently has hidden parameters that cannot be fixed when they become mal-set. Following a presentation of my evidence, I'll detail the issues & settings.

I've run Myst 3 with the following resolutions:
640x480 windowed
1280x720 windowed
1280x960 windowed
1280x720 (no scaling), black-matted in fullscreen 3840x2160
1280x960 (no scaling), black-matted in fullscreen 3840x2160
1920x1080 (no scaling), black-matted in fullscreen 3840x2160
7680x4320 (scaled), cropped to fullscreen 3840x2160

Details:
'myst3.ini' provides 3 directives that pertain:

widescreen_mod= // ... 'true' or 'false'
fullscreen= // ... 'true' or 'false' (false ==, windowed)
fullscreen_res= // ... example: '1280x720'

Especially note that there is no 'window_res=' directive (I tried it... doesn't work).

Rhetorical Question: If there is no 'window_res=' directive, how was I able to change 1280x960 windowed to 640x480 windowed? Well, I did this:

fullscreen=true
fullscreen_res=640x480 <--- previously was 1280x960

then ran the game. Then this:

fullscreen=false

then ran the game.

That put Myst 3 in a 640x480 window.
I have not been able to get it into a larger window since then.

That is not correct behavior.

Given the settings in 'myst3.ini', none of the resolutions I listed in paragraph #2 make any sense.

Further, this is screwing up Windows' feable (and awful) autoscaling.

Currently, my Myst 3 is
1280x720 (no scaling), black-matted,
Windows is not scaling,
the mouse 'finger' is too small to see on a 3840x2160 display, and consequently,
the game is unplayable.

I can't express how frustrated I am with this situation. I've been fighting this for days.

Regarding Windows scaling, here are the combinations -- I tried them all.

Program DPI
[_] Use this setting...
[I signed in to Windows]
[I open this program]

High DPI scaling override
[_] Override high DPI...
[Application]
[System]
[System (Enhanced)]

If you, dear reader, have any ideas, bring them out!

Warm Regards,
Mark.

I have an UPDATE

The problem was: Poor documentation leading to misunderstanding leading to bad configuration.

Background: My games run on an NVIDIA GPU. My laptop has a 2nd GPU: an Intel GPU, but I don't use it for games (or so I thought).

As it turns out, the NVIDIA connects only to the Display Port, on the side of the laptop. I don't use an external monitor. I use only the laptop's built-in display, and it's the Intel that connects to that. Apparently, this is the way most gaming laptops are engineered.

So, if the NVIDIA connects only to the Display Port but I don't connect an external monitor to the Display Port, then what good is having a fancy NVIDIA GPU?

Answer: Lots of good. Windows uses the NVIDIA as a graphical coprocessor to create images in the same manner that it uses an arithmetic coprocessor to do math. The only thing that the Intel GPU does is drive the built-in display and scale the desktop to fit it.

It's that second task that took me by surprise.

I figured that, because the game was 'running' on the NVIDIA GPU, I needed to go to the NVIDIA Control Panel to get it to scale fullscreen. When I didn't find any controls to do that, I thought my NVIDIA Control Panel was broken.

Well, the scaling settings are in the Intel Control Panel (which I never even looked at). I posted problems to Intel & Microsoft & NVIDIA forums. Everyone on those forums were mystified. The only documentation I could find talked only about multiple displays.

It was one technician at NVIDIA who gave me a clue that lead to the solution to my game scaling problem. It seems that everyone (except that one technician) practices NMP (not my problem).

For all the details, see https://answers.microsoft.com/en-us/windows/forum/games_windows_10/solved-how-can-i-get-change-high-dpi-settings-to/f073795f-8500-4b60-b76f-e838e7ba7227?tm=1536683457680