Hi,
I was reading alot on GoG forums and managed to get Diablo classic open in window to be able to alt tab and stream it.
I have moved:
dxcfg.exe
dxcfg.ini
ddraw.dll from dx to classic forlder, inserted virtual diablo cd and it works. Starts up in window mode, can alt tab etc. but ...
yes always but...
As you can see on screenshot - graphics and Battle net page are not synchronised.
Chat window and all other words are moved left top from graphics.

I had it same before copying files listed above, but simple could not see what was typed into spaces.
Now I see, but its in wrong place.
Any ideas ?
I have tried that dxcfg.exe in different combination but either its not that or can't find one.

I tried to move window to match them but it is impossible to move window

play using devilutionx to avoid all this crap

Game is playable as it was back in days.
Battle Net works, no graphical glitch in gameplay, just moved 'entry boxes' on battle net chat and login screen.
using dxcfg.exe and changing resolution made it visible on edge of screen.
I can live with that just was curious if anyone fixed it other way than moding whole game

cnc-ddraw can do it - https://github.com/FunkyFr3sh/cnc-ddraw/issues/214#issuecomment-1637008547

Don't think there is any other wrapper that can pull it off right now.

And why you think that? There is hundreds of them. Fork of each others mostly. Cnc not the first one and not so general one although become to. And best of them already included for DX version of GOG release.
Second best is simpliest ddra.dll one file solutions. Like older versions of Narzoul's Ddraw Compat Wrapper. As wrappers with some ini best one are GOG wrapper, and DxWnd, degevoodoo, WineD3D-ror-Windows and something called like DXcompat/D3Dcompat - rozen of them unrelated to eah others and each have dozen forks.

For Bnet multiplayer better avoid using any. So solutions is going to alternate versions Devulition/-X or just turn off crap software on your PC. Most of them better to be removed. General solution, also mentioned on Diablo GOG support page.
Any come with drivers but not part of them is first candidates to deletion. Nahimic, NV Experience, MSI Afteburner (and RivaTuner if its installed as part of MSI setup, they are NOT related and official download is separate from MSI), any input devices applications. It is not a drivers.

It is overlay app that cause it. Few overlay apps didnt even appear on screen and didnt looks like they are overlay apps. But they are. There is injectors and screen+window overtakers.

Borderless fullscreen windowed mode is another solution cause nowadays Windows and those overlay apps and nvidia drivers (they more issue than radeon for decade) made for such modes. But again it is DX or fanmade port variant, native Diablo do not support windowed.

You can check which one app inject to game process. Technically to do it you need tools bundle from Microsoft with Process Explorer in it. Sometimes Task Manager able to show some of interfering apps by RMB click on process (ofc in Details tab not in the main tab).
Anyway better close most of apps. And make sure they are closed by Details tab in Task Manager. Nowadays Windows not always really stop process when closing their windows/overlays.

If you still download and put to game folder ANY wrapper make sure you disable all compatibilities. There is hidden compatibility options pre-set on from GOG installer. I cant guarantee it is 100% work - looks like easier way to disable all at once is rename game exe to something like Game1.exe (with digit to avoid detecting it as a some random game by Windows and videodriver control panel).
Do not disabling them or using ini-based wrappers with too much high resolution can cause same effect too.

There is Verok's AppCompatibilityChecker tool but it cant remove most compatibilities set by GOG (those one setup by DISMM service and sdb-file script at the end of games install process). Can only show them and remove basic ones in one click.

His post was all about battle.net. cnc-ddraw is the only wrapper that can do windowed battle.net

You can Alt-Tab in Classic by hitting Ctrl-Alt-Tab.