Single Status Update
It's a miracle..!
So to recap: this breakable fixtures system I've been rambling about for the past days has two main components: a linedef that registers direct hitscan attacks and a nearby interactive mapspot that registers radius damage. The latter is done by the tried and tested method of giving this (invisible) mapspot a radius of 0, so that it becomes untargetable but still subject to blast damage.
EE only has the "impact" flag for linedef activation, not the "missile" one. That means that players wielding the plasma rifle are out of luck: the mapspot only registers blast damage; the linedef only hitscans and never projectiles. And I was fully prepared to handwave this by giving the usual spiel that "energy weapons don't shatter stuff", etc.
So I thought.
Until, during testing of E2M1, some breakable glass was hit by plasma fire and...shattered as a result. Wha..? Some debugging later, it turns out the mapspot is responsible for registering a projectile hit, despite the fact it is not / can not be targeted.
Don't get me wrong: I'm absolutely delighted that this works. But I have NO CLUE how this is even possible.
Unless Eternity has an equivalent to the VULNERABLE flag from ZDoom's Decorate, I think the only way for something that gets damaged by explosions but not projectiles is for it to have 0 height ... and I wouldn't be surprised if some ports ( or, heck, even vanilla ) allow projectiles to collide with 0 height actors, because the actor still has a z-height.
So, while I can't say I know what's happening from experience with Eternity, it's not improbable at all for the mapspot to be tall enough for projectiles be able to collide with and damage it. It would surprise me, since a mapspot having significant height is kinda weird to me, but if it's a type of mapspot specifically meant for reacting to projectiles and explosion, then it makes sense and isn't really strange.
Though it would seem more than a bit cumbersome to use, considering actor hitboxes are axis-aligned squares from the top-down in basically every source port, and a demo-compat source port changing that is a concept that's pretty bizarre to me.
Mapspot has 0 radius and 0 height, and yes this is specifically done to prevent it being targeted at all (so auto-aim does not lead your shots) while still being vulnerable to radius damage. It does not have Z-height, but is spawned at floor level. EE does have a "correct_height" Thing property, but the default here is also 0.
And yet, I can fire a plasma shot at the top of a 128 unit tall glass texture but it registers an actual collision with this 0 radius / 0 height mapspot sitting at floor level. Weird.
So weird in fact that I have never actually tested this before, assuming plasma just wouldn't work. Heh.