Here is the report:
Use of this function to create 3D effects is supposed to work but in reality has one or more bugs. This test map I created, [wiki]File:Testk.7z[/wiki], with the modifications I describe, demonstrates this occurrence. Note that the ground floor has a below-floor area adjoining a normal area, and exhibits no HOM, and also the passage between the demon's room and the invisible floor is a below-floor area between two normal areas, and shows no HOM. On the contrary, the stairway shows HOM (viewed from the stairs into the main room) on both the second and third floors, where one is looking resp. from a below-floor to a normal area and from a normal to an above-ceiling one; the other direction shows a solid wall (incorrectly). The passage on the third floor to the imp's room is interesting, as in the configuration it is in now, with all above-ceiling areas, it shows HOM when looking out from the main room, but only if one looks across all three lines dividing the two room (or both before I made the middle one). In a configuration where the imp's room, or that room and the hallway, are made normal sectors, the HOM shows up in the reverse direction only, and at the spot where one is looking from the normal to the above-ceiling area; the same is true in the right-hand room on the third floor if it is modified. Yet the staircase shows that it is not restricted to that case. Finally, there is currently a HOM from the demon's room back into the main room, though that shows up only when standing back of the diagonal line joining the slopes, and can also be seen in the mirror. Along this path there is only normal areas, and it showed up only when I constructed that room passing above and south of it, which should not affect it at all. This suggest a node-builder error, but when I removed the nodes from the wad (allowing my ZDoom (the latest version) to use its own node-builder), it did not change.
Therefore, there seem to be two problems with the 3D effects: one sometimes causing HOM when looking between areas with different height-transfer status, and one that can affect unrelated areas, probably due to the nodes being screwed up. Further these problems seem more likely to occur with fake ceilings than with fake floors, as when the center block (between the two ground-floor rooms) is elevated to the fake-ceiling region, it does show HOM at the usual spot. This proves a bug, as fake floors and fake ceilings should be treated exactly the same by the renderer. This is not an invariable rule though as even when my test map is edited to remove the whole third story and all use of false ceilings, the bug on the stairs remains; however, it must apply in normal situations, because many test and production levels have used the technique with false floors to produce 3D geometry.
A second issue related to 3D shown by my test level is that the lift in the main room, when it rises above the fake floor, becomes invisible. More generally, a real floor or ceiling can't move through a fake floor or ceiling and be rendered properly. This isn't technically a bug, since it is intended behavior, but it it rather pointless since the invisible-floor effect it was meant to create (in Boom) is rare and now more easily handled by the bridge thing. I can't see why the renderer couldn't recognise which section the lift (etc.) is in now and render it there, and the old behavior if really needed still emulated for compatibility.
Finally, although possibly not related, another bug shown by my demonstration relates to the teleporter line. If you can activate it from the upper level you can get stuck in a wall, though it may take some trying to reproduce this.
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