ZDoom

Use linedef 242 instead of self referencing sectors. Those will block monster sight.

Oh damn, I didn't realize this was a bug. >_< One of my Boom-compatible maps uses self-referencing sectors to block monster sight (though it's not too late to change it, as I haven't released it publicly yet) - I was wondering why it wasn't happening in PrBoom+.

I suppose I might have to dabble in some complex REJECT witchcraft to get the effect I want - unless there's a simpler way of doing it that I'm not aware of.

Expanded the compat_trace option to handle this case, too. Now all I need is a list of maps which needs this option set.

Not sure if this is in abolutely all versions, but it has at least been in several versions before the current one. (Besides, the title is max length already.)

There are some maps in Eternal Doom where the level attempts to start monster infigting by having monsters attempting to shoot the player through a self-referencing sector.
It's noticeable on Map14: Pistol only, where there are a lot of monsters guarding the blue key that are supposed supposed to kill each other, but won't start shooting, or even waking up, until you're on the same side of the self-refencing sector.
The problem also appears in the "rocket armor" part of Map26: No parking, where the three Cyberdemons are supposed to shoot at the mancubi and barons of hell until the cyberdemon in the middle dies so you can shhot the button behind him.

The bug is noticeable in a lot of places where the "self-refencing sectors don't block shots" compatibility option needs to be activated, as the monsters will never shoot at you even when you have a clear shot at them, but the two incidents in Eternal Doom are the main points where it's actually made it difficult to complete the level.