[All Source games] Looking at the corner of the skyboxes will cause the corner to flicker and show little bit of clip plane

[megakarlach]

When looking at the corner of the skybox, the corner would flicker and show little bit of the black/white clip plane.

Seems to affect all Source engine games (currently confirmed on CS:S, L4D2, HL2 and HL2DM), but it was more noticeable on Half-Life 2 (since 20th anniversary update, display as black plane) and HL2DM (even without zooming, display as white plane), while in CSS and L4D2, it's only noticeable when you zoom in using weapons like AWP.

I also tested this on all other PCs (including different graphics card like Intel HD) that I have and they all have the same issues.

Image:
(Without zooming in)

(Zooming in)

(L4D2, using AWP)

Video:
Counter-Strike: Source

CSS.corner.bug.mp4

Half-Life 2

hl2_corner_20th.mp4

Half-Life 2: Deathmatch

HL2DM.skybox.corner.bug.mp4

System information:
Tested on all PCs (with Nvidia GPU) and a laptop (with Intel HD 3000)
My (main) PC: (i5-12400F, RTX 3060 12GB, 32GB RAM) - https://gist.github.com/megakarlach/fc0d42988ccc48de24d0419f81cb5a1c
My second PC: (i5-3470S, GTX 750Ti, 8GB RAM) - https://gist.github.com/megakarlach/fdd7b29ef42e2995e40c7fda95ef9040
Not GPU-related since I also tested this on a laptop with Intel HD 3000.

[BreakinBenny]

Oh no...

[dimhotepus]

Skybox culled incorrectly when high FOVs.

#2657

Possible duplicate?