Single Status Update
I sincerely doubt I am the first person who has discovered this, but I didn't find any posts where anybody elaborated on this subject, or any mention of this in any of the related wikis.
Let's start at the beginning. I've always been irritated by the seemingly impossible prospect of creating a swinging door in a DooM mod. Short of over-complicated level alteration shenanigans, the only other way of doing it was with polyobjects. However, at the time I was not aware of the two-sided linedef trick that allows you to at least have the ILLUSION of a limited sized door, as opposed to the infinitely tall polyobjects I dabbled with and gave up on before.
After I found out about this, and since I mostly make maps using the UDMF format, I got to thinking about whether or not the walkable middle texture flag would work on a polyobject's textures. Suffice to say, it did, and it made me happy. This is not the thing I'm wondering if others have discovered. The polyobject door now swung and clipped properly, but you could still shoot through it. There was no "bullets clip only at the texture flag", so once again the only option was for projectiles to clip anywhere vertically where the line was. Making progress, but still I dabbled.
I started wondering if I could somehow combine GZDoom's 3D floors with polyobjects, essentially making a dummy sector withing a dummy sector. If it worked, I'd have a door that clipped all things properly. It was doubtful that it'd work, but I tried it anyway. The results were not FULLY what I'd hoped, but it was a happy surprise anyway.
What I ended up with was an invisible door. There was no visual, but the 3D floor's collision data was being transferred to the polyobject. I could walk on it, my bullets and I collided with the invisible door, and it even showed up on the map.
The reason why the doors didn't have a visual still didn't quite make sense to me, but at the same time it sort of did. Since in the actual map data there isn't a real physical object there to provide the polyobject with texture, I can sort of see why it doesn't; but if I clip into the dummy sector, the 3D floor's sides are missing. So where'd they go?
Now if I combine this with the two-sided linedef trick, I can give the door's sides texture, and decals even show up on it. A much better position to be at than where I was a while ago. I still don't have a top texture on my door, but due to the way polyobjects are handled, it strikes me as being unavoidable, unless the ZDoom developers add some extra functionality.
Like I said, I'm an out of touch dude when it comes to this community, so I have to figure that SOMEONE out there has already discovered this, and maybe even posted about it. I'd like to know if somebody has managed to find how to lift the visual limitations of the 3D floor polyobject, if that's even possible, or what other tricks there are in this vein.
Even if I'm not the first to find out about this, it sure was super exciting to discover it on my own. Eeeeeee!