[BUG] Invisible entity gets created and COLLIDES with other entities (like prop_physics) when a player enters a seat

[Dave9031]

Short Description
Hello, i have recently returned to gmod and i have noticed some weird collisions with seemingly nothing when i have ACF-3 installed and I enter a built vehicle and start driving.
It seems there is an invisible entity being created which falls to the ground and collides with other entities when a player enters a seat
(this does not seem to happen if the vehicle is for example controlled via wiremods remote control, but only when a player enters a seat)

Branch and Version
Newest - Git-master-b4254b4

How to Reproduce
It seems that all you need to reproduce this is any "ACF whitelisted" seat (hl2 seats, playerstart seats, race seats) - does not happen on jeeps, airboats
I found it easiest to reproduce by building a car and adding a prop as a plow infront of the seat
-- Please check the attached video --

However, even just a prop_physics with 400kg mass and a thruster with the seat welded infront of it works
--> Enter the seat, make the prop move (even just thrusters work) into the "spawned entity" (it seems to spawn and hang around below the seat, on the ground)
--> Collision sound can be heard

Video
I have attached a rather simplistic video demonstrating the issue, but i believe it shows everything that is needed
I have used a prop engine car with a friction gearbox, since the amount of props makes it easier to see the collisions
https://github.com/user-attachments/assets/2f98674c-164d-4bc9-a955-3a8d21754218
(I know the quality is not great since its compressed to fit githubs 10MiB limit)

(Im probably ovethinking by time stamping this ... oh well lol)
0:05 - I drive past the "entity" and it collides with the gearbox - collision noises can be heard
0:12 - This time I drive backwards - it collides with the "plow" infront - collision noises can be heard
0:15 - This time it collides even with the wheels - collision noises can be heard
0:31 - I test that going backwards if fine - "entity" should be infront
0:56 - Totally fine if the seat is out of the way - and therefore the "entity" is too
1:04 - Purposely drive into it - yep its there

Additional Information
I have quite barbarically searched for the root of the issue (by adding prints of course! :) ) and I hopefully narrowed it down to the "seat alias" system
#432 Might be related to this

ACF-3/lua/entities/acf_seat_alias/init.lua

hook.Add("PlayerEnteredVehicle", "ACF.CreateSeatAlias", function(Ply, Vic)
    if not IsValid(Ply) then return end
    if not IsValid(Vic) then return end

    print("Applying alias "..tostring(Ply).."/"..tostring(Vic)) -- heh
    ACF.ApplyAlias(Vic, Ply) -- When I barbarically comment this out the problem seems to go away
end)

I think the main issue could be here :

ACF-3/lua/entities/acf_seat_alias/init.lua

function MakeACF_SeatAlias(Vehicle) -- Called by ACF.ApplyAlias
...
  Ent:PhysicsInit(SOLID_VPHYSICS)
  Ent:SetMoveType(MOVETYPE_NONE)
  
  Ent:SetParent(Vehicle) -- This does not seem to work since the "entity" seems to hang around on the ground?
  Ent:SetPos(Vehicle:LocalToWorld(AliasInfo.Pos))
  Ent:SetAngles(Vehicle:LocalToWorldAngles(AliasInfo.Ang))
  Ent:Spawn()
  
  Ent:SetCollisionGroup(COLLISION_GROUP_NONE)
...
end

[OctothorpeObelus]

This is because the collision group is set to none, which is the normal collision group for all objects, not a collision group that means no collisions. It has yet to be fixed. You can patch it locally by changing the collision group to COLLISION_GROUP_WORLD which does not collide with players or physics entities.

[thecraftianman]

This will no longer be a problem due to the seat alias system as it stands now being removed in 6bada10.