All anchored portals in an area must be at the same height

[printz]

There seems to be a design limitation here: if you want to make an area multi-story, you need to cut the entirety of it with a portal. You can't have ceiling portals that change height from sector to sector, unless you're OK with having floating solid textures. In order to look correct, the portal plane needs to be unique. You cannot have a horizontal cut that stops at the sector linedefs and then continues vertically along each line.

This makes designs such as 3D stairs very wasteful with linked portals, since you basically need a new portal with every step.

I mean that designs like these are impossible right now:

+----------------+------------------------------+
|                .                              |
|         portal .portal mid wall               |
|     upper wall .                portal floor  |
|                +..............................+
|                               portal ceiling  |
|                                               |
|     normal passable mid line                  |
+----------------+------------------------------+

If it worked, it would save a lot of mapping effort, by not affecting the left part. Right now you have to cut the left part too, affecting all the detail that's already there, and the cut would have to extend until it encounters a wall.

[Graf Zahl]

I have no idea how complex this would be with Eternity's renderer but imagining how this might affect how the renderer iterates through the portal I think it can easily devolve into serious issues.

Just in your simple case imagine a view from the bottom right to the top left which starts in section 1, passes through section 2 and ends up in section 1 yet again. I only can tell from how this would work in GZDoom's hardware renderer, but there it'd lead to some highly non-trivial recursion issues that are basically unsolvable, which is compounded by the wall portal only being a single tier of the wall, not the entire height.

Having a single plane as separator certainly makes this a lot easier to handle.