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.