- when adding some minmum lateral movement to trigger collision detection, do not just set Vel.X but actually use a vector pointing in the proper facing direction of the actor.

src/playsim/p_mobj.cpp

@@ -3885,7 +3885,7 @@ void AActor::Tick ()
 		// (for backwards compatibility this must check for lack of damage function, not for zero damage!)
 		if ((flags & MF_MISSILE) && Vel.X == 0 && Vel.Y == 0 && !IsZeroDamage())
 		{
-			Vel.X = MinVel;
+			VelFromAngle(MinVel);
 		}
 
 		// Handle X and Y velocities

wadsrc/static/zscript/actors/shared/fastprojectile.zs

@@ -87,7 +87,7 @@ class FastProjectile : Actor
 			// force some lateral movement so that collision detection works as intended.
 			if (bMissile && Vel.X == 0 && Vel.Y == 0 && !IsZeroDamage())
 			{
-				Vel.X = MinVel;
+				VelFromAngle(MinVel);
 			}
 
 			Vector3 frac = Vel / count;