I've turned the advanced switch off for topmost as instructed on the win10 known issues post, but before i did, I was able to notice something about this problem...

When changing resolutions from say... 4k down to 1366x768... if topmost is enabled, windows that were not already inside the 1366x768 confined area before the resolution change away from 4k are not moved into the 1366x768 resolution container (meaning that are still being rendered outside the smaller viewing area.)

turning topmost off, causes the windows to all be successfully moved, and resized to fit inside the new smaller resolution. <-- when changing resolutions from the settings app, this is done by windows 10 automatically.

A work around, with topmost enabled (which it really should be), upon resolution change on a single display (or when switching from 1 display to another), have DF resize all windows, and then re-position them all to 0x0 (top left corner of the display.)

The windows are there, they are just not on the screen in these instances, because they didn't appear to actually be relocated.

figured this out, when some windows wouldn't appear after trying to bring them to the front, but noticed (for some of them) that I could move them manually back onto the screen. for others, they were partially on screen, and partly off screen, (because they were already partly in the smaller resolution's viewing area before switching), which I could easily move. Also none of the windows is ever resized to fit the new smaller resolutions, if topmost is enabled

Disabling topmost fixes this issue, but removes the topmost intention of windows 10.

Ultimately, this sounds like a window location issue caused by topmost. more specifically, DF isn't moving/resizing windows to match resolution changes, any time topmost is enabled. I hope this helps you! I'll work with you as I can.