Quick question

How do I get a floor/ceiling to stretch to the horizon in Vanilla Doom?
I have a window area, and I've been playing around with heights and missing textures and such, but I haven't quite gotten it.

I'm not sure what you mean. buttah...

Make a large sector behind the window (floor height=0 / ceiling height=128 for example). Stretch the sector to the point you want your horizon to be.

Make another sector behind that one of which the floor and ceiling height are identical (floor height=0 / ceiling height=0). Don't use an upper texture. Set the the Ceiling textures of both sector to F_sky1.

Ah, no... I know how to "put a sky on a wall"-- but I thought there was a trick that allowed, say, a water texture to repeat infinitely into the horizon.

You mean have a sky flat on the ceiling and a water flat on the ground?

I thought you wanted the "infinite sky" trick...

This is no "infinite sky trick" in vanilla Doom. Unless you discover it :-P

I saw the effect in the first level of Skulltag DM. So you're saying the trick is source port specific?

Yeah. Only ZDoom, Skulltag and ZDaemon (and maybe Eternity?) can do that. It's linedef action 9 in Doom in Hexen Format.