ZDoom

It could be that it is because the height is not a power of two - I don't really know.

It is true that this occurs in Heretic and Hexen but not in Doom. May I ask what is the difference? Is it because skies in Doom are 128 px high and skies in Heretic/Hexen are 200 px high?

Another question - if I make completely new project that won't use any assets from retail games, am I able to create a sky texture that will fill the entire screen when looking up and down (no capping)?

This particular bug isn't that likely to get fixed as certain features in the software renderer makes it difficult to figure out why the texture is about to repeat.

Hmm, yes it seems the Capped sky render method doesn't work with tall skies in the Carmack renderer. In Heretic the sky clearly tiles when looking up, while in Doom and Doom 2 it works as expected. The SoftPoly and OpenGL renderers are fine in all cases. I'd say this should be reported as a bug (if it isn't already).

The thing is this doesn't work anymore. I have a sky that is 200 px high. When I look up enough to have the upper border of thy sky at the upper edge of the screen the fade out effect will show up. But when I look up even more the fade out effect disappears and the sky will tile. I was hoping that there is some workaround.

Under Display Options -> Software Renderer, you can change the Sky Render Mode to Capped to prevent vertical tiling when looking up. It'll generate a color given the top of the texture and fill the area above the image with that.

Is there a way to make sky texture that will not tile in software mode when looking up (assuming that you're using extended up limit)? I have tried to find a solution and found some old info about how it is broken but don't know if it's true or not.

There is this wiki entry but seems obsolete: https://zdoom.org/wiki/Sky_stretching

The thing is that if I create sky texture that is higher than 200 px it will shift down and the tiling will occur.