ZDoom

Looking at window midtextures while in Truecolor Software on a map that has fog, you see just pure black in the windows instead of what's supposed to be visible on the other side.
Comparison screenshots:

Getting very close to the window sometimes removes the blind and allows normal view. Also, tilting at particular angles partially opens the black blind, as you can see in the Truecolor side view image.

I took the screenshots in plain Hexen (MAP08, no mods) but I have also seen this in other PWADs with fog and windows, so not Hexen-specific.
Screenshots are in 640x480 in Vulkan, but the blinds behave exactly the same at higher resolutions (including widescreen) and with OpenGL too.
I'm running an Nvidia GTX 970 on Windows 7.
This is GZDoom 4.7.1 but it's also there in 4.5.0.

I've been replaying Hexen recently, and noticed this glitch isn't limited to windows, it's also very prominent in the portals.
As before, this black film is specific to Truecolor (hardware and regular software renderers look fine), it's there regardless of the API used (OpenGL, GLES, Vulkan; this time I also tested SoftPoly, it's there in all of them).
I'm starting to wonder if this could be a problem specific to my setup and not to GZDoom, it's such an obvious glitch but I haven't found any record of it being reported anywhere before. If anyone else can confirm they have the same issue (or conversely, that they do not) I'd appreciate it.

Can confirm this happens to me too on the latest devbuild. Winnowing Hall is fine, while Darkmere shows a black portal window with true colour SW renderer. Interestingly, the save game thumbnail works just fine; is the non-TC renderer used for those?

Figured I would answer your question since the post is at the top anyway as of may 28th 2022. Save games are really just pngs. Part of it is the actual screenshot, and the other is just a bunch of binary data. That's why you cant edit your saves with a save game editor. Not to mention they don't exist, because it really amounts to a hacked PNG.

Savegames haven't been PNGs for many years, these days they are zips with a PNG screenshots and a few JSON files.

Oh, it seems like the source I read from was outdated.