Fixing "walking down stairs shows falling message" (#47588)

* I'm too tired to even PR it atm.

* Update code/game/turfs/turf.dm

Co-Authored-By: Emmett Gaines <ninjanomnom@gmail.com>

* Update code/game/turfs/turf.dm

Co-Authored-By: Emmett Gaines <ninjanomnom@gmail.com>

* Update code/game/turfs/turf.dm

Co-Authored-By: Emmett Gaines <ninjanomnom@gmail.com>

* atom signal, not movable.

code/__DEFINES/components.dm

@@ -121,6 +121,9 @@
 #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)
@@ -144,7 +147,6 @@
 #define COMSIG_TURF_HAS_GRAVITY "turf_has_gravity"				//from base of atom/has_gravity(): (atom/asker, list/forced_gravities)
 #define COMSIG_TURF_MULTIZ_NEW "turf_multiz_new"				//from base of turf/New(): (turf/source, direction)
 
-
 // /atom/movable signals
 #define COMSIG_MOVABLE_PRE_MOVE "movable_pre_move"					//from base of atom/movable/Moved(): (/atom)
 	#define COMPONENT_MOVABLE_BLOCK_PRE_MOVE 1

code/__DEFINES/misc.dm

@@ -463,3 +463,8 @@ GLOBAL_LIST_INIT(pda_styles, sortList(list(MONO, VT, ORBITRON, SHARE)))
 #define BAD_ART 12.5
 #define GOOD_ART 25
 #define GREAT_ART 50
+
+// 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.

code/game/atoms.dm

@@ -1177,7 +1177,7 @@
 		return filters[filter_data.Find(name)]
 
 /atom/proc/intercept_zImpact(atom/movable/AM, levels = 1)
-	return FALSE
+	. |= SEND_SIGNAL(src, COMSIG_ATOM_INTERCEPT_Z_FALL, AM, levels)
 
 ///Sets the custom materials for an item.
 /atom/proc/set_custom_materials(list/materials, multiplier = 1)

code/game/objects/structures/stairs.dm

@@ -111,7 +111,9 @@
 			T.ChangeTurf(/turf/open/openspace, flags = CHANGETURF_INHERIT_AIR)
 
 /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/turf.dm

@@ -141,12 +141,19 @@
 /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
-	if(zFall(A, ++levels))
+		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, src))
 		return FALSE
 	A.visible_message("<span class='danger'>[A] crashes into [src]!</span>")
 	A.onZImpact(src, levels)
@@ -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

@@ -686,7 +686,9 @@ 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))