I'm just going to post a new one on this, because I'm having trouble finding one in the last year or so talking about this on windows 10, but the issue I'm having is very similar to one of the issues discussed in https://www.displayfusion.com/Discussions/View/windows-10-windows-snapping-a-few-pixels-off/?ID=8dae7bc8-1e10-44dc-9e39-fb3887f18aae

When using the Size and Move Window functions, there are gaps on the left and right of a lot of windows when multiple windows are being tiled together with those functions. It doesn't happen with all of my apps, but it has been happening with most apps I've been trying (including Chrome, Firefox, and all of the Windows system apps I've tried. (settings, explorer, etc.)

It doesn't happen with the Steam, Twitch, or Discord app. All of those seem to tile correctly, and will do so even if other windows have these gaps. (Or more accurately, it will leave a smaller gap if it is tiled with a window that is leaving gaps, correctly occupying half the monitor) There is also no gap between on the far right or far left edge of my monitor setup, but there is a gap between neighboring monitors.
The Windows snap shortcuts aren't leaving gaps with any of the windows.

I have a 3840x2160 monitor centered between two portrait 1920x1200 monitors on Windows 10.

That's been my understanding, but how are you dealing with the edges of the draw space? The windows are still flush to the monitor and I can see that the coordinates for those windows extends beyond the screens, either starting at coordinates beyond the draw space or being slightly too big to actually fit.

And relatedly, has anyone found a way to turn those borders *off* yet? They sound like they'd be annoying for more than just this program.

No problem, have a debug file!