ZDoom

Actually, the issue is reproducable. Just start 'gzdoom -iwad doom2.wad +map map02' and it'll start with a black screen until you open the menu. This only happens with the regular GL backend.

Unfortunately, we're unable to diagnose the issues directly as we don't have access to the specific hardware. However, a good starting point would be to experiment with your drivers by switching between different versions to see if that resolves the issue. Although I wish I could be of more help, these steps are the best we can suggest under the circumstances.

gzdoom.exe -skill 4 -warp 1 +vid_rendermode 4 +vid_preferbackend 0

gzdoom.exe -skill 4 -warp 1 +vid_rendermode 1 +vid_preferbackend 0

gzdoom.exe -skill 4 -warp 1 +vid_rendermode 0 +vid_preferbackend 0

(You might not bother with this?) This happens in Intel and AMD iGPU laptops and in a rig with a NVidia card. Both in fullscreen or in a window.

The black screen appears always when GZDoom is run by warpipng directly to a map, with Opengl renderer backend and the rendermode either Hardware accelerated or True Color. In Opengl/True color the new loading bar is left on the otherways black screen. The black screen goes away by pressing ESC to display the menu screen.

It doesn't happen in Opengl with Doom software renderer.
It doesn't happen in Vulkan or Opengl ES in any rendermode.
If GZDoom is run without warping to a map the titlescreen displays fine.

It happens with:
gzdoom-x64-g4.12pre-516-g7a1342188 (the latest)
gzdoom-x64-g4.12pre-362-g1c7f19535 (Jan 18th)

but not with
gzdoom-x64-g4.12pre-214-g273f7c55b (Dec 18th)
and 4.11.3.

Commandlines for Doom2.wad:

Black screen: Opengl + Harware accelerated:
[code]gzdoom.exe -skill 4 -warp 1 +vid_rendermode 4 +vid_preferbackend 0[/code]

Black screen w/ brown loading bar: Opengl + True color sw renderer:
[code]gzdoom.exe -skill 4 -warp 1 +vid_rendermode 1 +vid_preferbackend 0[/code]

No black screen: Opengl + Doom sw renderer
[code]gzdoom.exe -skill 4 -warp 1 +vid_rendermode 0 +vid_preferbackend 0[/code]