I encountered a minor simulation oddity that shouldn't really happen. CO2 is a gas, right? It shows up in the gas menu and behaves like a gas, albeit a heavy one. Then why does it fall through powder walls?

Just a little thing I noticed.

EDIT: Users jasiu2002 and Qwert_Yuiop discovered that fire and smoke do it too, which is hilarious.

EDIT 2: Okay, there's been some speculation on the save comments about the cause of this, so I might as well explain it here where it'll be a lot easier. Here's some code:

Collision logic:

if (bmap[ny/CELL][nx/CELL]==WL_ALLOWSOLID && elements[pt].Falldown!=1)

//collide

Placement logic:

else if (wall == WL_ALLOWSOLID && elements[type].Falldown!=1)

//don't allow placement

This is the code in Simulation.cpp, from lines 1862 and 2068, respectively. What it does is check what kind of wall is at a point (the wall == WL_ALLOWSOLID or bmap[ny/CELL][nx/CELL] == WL_ALLOWSOLID bits), then determine the kind of particle passing through (the elements[type].Falldown!=1 bit). If both are true, this interaction is flagged as being collision or invalid placement.

What this means is that TPT isn't checking the state of a particle (which does appear to be defined separate from the falldown value), just the falldown value. Curiously, SMKE, FIRE, and CO2 all have a falldown value of 1, which seems to have no effect (I tested it). Since the if statement checks that the falldown value is NOT equal to 1, when CO2, FIRE, or SMKE pass through, that little not-equal-to statement doesn't evaluate to true, meaning that a collision or invalid placement is not registered.