PR #47500, #47501, #47557, #47588.

code/__DEFINES/components.dm

@@ -77,6 +77,10 @@
 #define COMSIG_ATOM_ATTACK_HAND "atom_attack_hand"				//from base of atom/attack_hand(): (mob/user)
 #define COMSIG_ATOM_ATTACK_PAW "atom_attack_paw"				//from base of atom/attack_paw(): (mob/user)
 	#define COMPONENT_NO_ATTACK_HAND 1							//works on all 3.
+//This signal return value bitflags can be found in __DEFINES/misc.dm
+#define COMSIG_ATOM_INTERCEPT_Z_FALL "movable_intercept_z_impact"	//called for each movable in a turf contents on /turf/zImpact(): (atom/movable/A, levels)
+
+
 /////////////////
 
 #define COMSIG_ENTER_AREA "enter_area" 						//from base of area/Entered(): (/area)

code/__DEFINES/misc.dm

@@ -511,5 +511,10 @@ GLOBAL_LIST_INIT(pda_reskins, list(PDA_SKIN_CLASSIC = 'icons/obj/pda.dmi', PDA_S
 #define VOMIT_TOXIC 1
 #define VOMIT_PURPLE 2
 
+// possible bitflag return values of intercept_zImpact(atom/movable/AM, levels = 1) calls
+#define FALL_INTERCEPTED		(1<<0) //Stops the movable from falling further and crashing on the ground
+#define FALL_NO_MESSAGE			(1<<1) //Used to suppress the "[A] falls through [old_turf]" messages where it'd make little sense at all, like going downstairs.
+#define FALL_STOP_INTERCEPTING	(1<<2) //Used in situations where halting the whole "intercept" loop would be better, like supermatter dusting (and thus deleting) the atom.
+
 //Misc text define. Does 4 spaces. Used as a makeshift tabulator.
 #define FOURSPACES "&nbsp;&nbsp;&nbsp;&nbsp;"

code/game/atoms.dm

@@ -839,4 +839,4 @@ Proc for attack log creation, because really why not
 		return TRUE
 
 /atom/proc/intercept_zImpact(atom/movable/AM, levels = 1)
-	return FALSE
+	. |= SEND_SIGNAL(src, COMSIG_ATOM_INTERCEPT_Z_FALL, AM, levels)

code/game/objects/structures/stairs.dm

@@ -2,14 +2,17 @@
 #define STAIR_TERMINATOR_NO 1
 #define STAIR_TERMINATOR_YES 2
 
+// dir determines the direction of travel to go upwards (due to lack of sprites, currently only 1 and 2 make sense)
+// stairs require /turf/open/openspace as the tile above them to work
+// multiple stair objects can be chained together; the Z level transition will happen on the final stair object in the chain
+
 /obj/structure/stairs
 	name = "stairs"
 	icon = 'icons/obj/stairs.dmi'
 	icon_state = "stairs"
 	anchored = TRUE
-	//dir = direction of travel to go upwards
 
-	var/force_open_above = FALSE
+	var/force_open_above = FALSE // replaces the turf above this stair obj with /turf/open/openspace
 	var/terminator_mode = STAIR_TERMINATOR_AUTOMATIC
 	var/turf/listeningTo
 
@@ -108,7 +111,9 @@
 			T.ChangeTurf(/turf/open/openspace)
 
 /obj/structure/stairs/intercept_zImpact(atom/movable/AM, levels = 1)
-	return isTerminator()
+	. = ..()
+	if(isTerminator())
+		. |= FALL_INTERCEPTED | FALL_NO_MESSAGE
 
 /obj/structure/stairs/proc/isTerminator()			//If this is the last stair in a chain and should move mobs up
 	if(terminator_mode != STAIR_TERMINATOR_AUTOMATIC)

code/game/turfs/openspace/openspace.dm

@@ -4,8 +4,6 @@
 	icon_state = "grey"
 	baseturfs = /turf/open/openspace
 	CanAtmosPassVertical = ATMOS_PASS_YES
-	plane = FLOOR_OPENSPACE_PLANE
-	layer = OPENSPACE_LAYER
 	//mouse_opacity = MOUSE_OPACITY_TRANSPARENT
 	var/can_cover_up = TRUE
 	var/can_build_on = TRUE
@@ -14,8 +12,10 @@
 	..()
 	return TRUE
 
-/turf/open/openspace/Initialize()
+/turf/open/openspace/Initialize() // handle plane and layer here so that they don't cover other obs/turfs in Dream Maker
 	. = ..()
+	plane = FLOOR_OPENSPACE_PLANE
+	layer = OPENSPACE_LAYER
 	return INITIALIZE_HINT_LATELOAD
 
 /turf/open/openspace/LateInitialize()

code/game/turfs/turf.dm

@@ -141,11 +141,18 @@
 /turf/proc/zAirOut(direction, turf/source)
 	return FALSE
 
-/turf/proc/zImpact(atom/movable/A, levels = 1)
+/turf/proc/zImpact(atom/movable/A, levels = 1, turf/prev_turf)
+	var/flags = NONE
+	var/mov_name = A.name
 	for(var/i in contents)
 		var/atom/thing = i
-		if(thing.intercept_zImpact(A, levels))
-			return FALSE
+		flags |= thing.intercept_zImpact(A, levels)
+		if(flags & FALL_STOP_INTERCEPTING)
+			break
+	if(prev_turf && !(flags & FALL_NO_MESSAGE))
+		prev_turf.visible_message("<span class='danger'>[mov_name] falls through [prev_turf]!</span>")
+	if(flags & FALL_INTERCEPTED)
+		return
 	if(zFall(A, ++levels))
 		return FALSE
 	A.visible_message("<span class='danger'>[A] crashes into [src]!</span>")
@@ -161,11 +168,10 @@
 		return FALSE
 	if(!force && (!can_zFall(A, levels, target) || !A.can_zFall(src, levels, target, DOWN)))
 		return FALSE
-	A.visible_message("<span class='danger'>[A] falls through [src]!</span>")
 	A.zfalling = TRUE
 	A.forceMove(target)
 	A.zfalling = FALSE
-	target.zImpact(A, levels)
+	target.zImpact(A, levels, src)
 	return TRUE
 
 /turf/proc/handleRCL(obj/item/twohanded/rcl/C, mob/user)

code/modules/power/supermatter/supermatter.dm

@@ -651,7 +651,9 @@ GLOBAL_DATUM(main_supermatter_engine, /obj/machinery/power/supermatter_crystal)
 	Consume(AM)
 
 /obj/machinery/power/supermatter_crystal/intercept_zImpact(atom/movable/AM, levels)
+	. = ..()
 	Bumped(AM)
+	. |= FALL_STOP_INTERCEPTING | FALL_INTERCEPTED
 
 /obj/machinery/power/supermatter_crystal/proc/Consume(atom/movable/AM)
 	if(isliving(AM))