Prevent a source of MC failures. (#28169)

code/__DEFINES/dcs/movable_signals.dm

@@ -81,8 +81,8 @@
 #define COMSIG_MOVABLE_DRIFT_BLOCK_INPUT "movable_drift_block_input"
 	#define DRIFT_ALLOW_INPUT (1<<0)
 
-///called when the movable's glide size is updated: (new_glide_size)
-#define COMSIG_MOVABLE_UPDATE_GLIDE_SIZE "movable_glide_size"
+///called after the movable's glide size is updated: (old_glide_size)
+#define COMSIG_MOVABLE_UPDATED_GLIDE_SIZE "movable_glide_size"
 
 ///signal sent out by an atom when it is no longer pulling something : (atom/pulling)
 #define COMSIG_ATOM_NO_LONGER_PULLING "movable_no_longer_pulling"

code/datums/components/drift.dm

@@ -90,7 +90,7 @@
 	RegisterSignal(movable_parent, COMSIG_MOVABLE_MOVED, PROC_REF(handle_move))
 	// We will use glide size to intuit how long to delay our loop's next move for
 	// This way you can't ride two movements at once while drifting, since that'd be dumb as fuck
-	RegisterSignal(movable_parent, COMSIG_MOVABLE_UPDATE_GLIDE_SIZE, PROC_REF(handle_glidesize_update))
+	RegisterSignal(movable_parent, COMSIG_MOVABLE_UPDATED_GLIDE_SIZE, PROC_REF(handle_glidesize_update))
 	// If you stop pulling something mid drift, I want it to retain that momentum
 	RegisterSignal(movable_parent, COMSIG_ATOM_NO_LONGER_PULLING, PROC_REF(stopped_pulling))
 
@@ -99,7 +99,7 @@
 	var/atom/movable/movable_parent = parent
 	movable_parent.inertia_moving = FALSE
 	ignore_next_glide = FALSE
-	UnregisterSignal(movable_parent, list(COMSIG_MOVABLE_MOVED, COMSIG_MOVABLE_UPDATE_GLIDE_SIZE, COMSIG_ATOM_NO_LONGER_PULLING))
+	UnregisterSignal(movable_parent, list(COMSIG_MOVABLE_MOVED, COMSIG_MOVABLE_UPDATED_GLIDE_SIZE, COMSIG_ATOM_NO_LONGER_PULLING))
 
 /datum/component/drift/proc/before_move(datum/source)
 	SIGNAL_HANDLER
@@ -147,7 +147,7 @@
 /// We're going to take the passed in glide size
 /// and use it to manually delay our loop for that period
 /// to allow the other movement to complete
-/datum/component/drift/proc/handle_glidesize_update(datum/source, glide_size)
+/datum/component/drift/proc/handle_glidesize_update(datum/source, old_glide_size)
 	SIGNAL_HANDLER
 	// If we aren't drifting, or this is us, fuck off
 	var/atom/movable/movable_parent = parent
@@ -158,7 +158,7 @@
 	if(ignore_next_glide)
 		ignore_next_glide = FALSE
 		return
-	var/glide_delay = round(world.icon_size / glide_size, 1) * world.tick_lag
+	var/glide_delay = round(world.icon_size / movable_parent.glide_size, 1) * world.tick_lag
 	drifting_loop.pause_for(glide_delay)
 	delayed = TRUE
 

code/game/atoms_movable.dm

@@ -743,8 +743,12 @@
 
 /// This proc is recursive, and calls itself to constantly set the glide size of an atom/movable
 /atom/movable/proc/set_glide_size(target = 8)
-	SEND_SIGNAL(src, COMSIG_MOVABLE_UPDATE_GLIDE_SIZE, target)
+	if(glide_size == target)
+		return
+
+	var/old_value = glide_size
 	glide_size = target
+	SEND_SIGNAL(src, COMSIG_MOVABLE_UPDATED_GLIDE_SIZE, old_value)
 
 	for(var/mob/buckled_mob as anything in buckled_mobs)
 		buckled_mob.set_glide_size(target)

code/modules/mob/mob_grab.dm

@@ -47,7 +47,7 @@
 	affecting.grabbed_by += src
 	RegisterSignal(affecting, COMSIG_MOVABLE_MOVED, PROC_REF(grab_moved))
 	RegisterSignal(assailant, COMSIG_MOVABLE_MOVED, PROC_REF(pull_grabbed))
-	RegisterSignal(assailant, COMSIG_MOVABLE_UPDATE_GLIDE_SIZE, PROC_REF(on_update_glide_size))
+	RegisterSignal(assailant, COMSIG_MOVABLE_UPDATED_GLIDE_SIZE, PROC_REF(on_updated_glide_size))
 
 	hud = new /atom/movable/screen/grab(src)
 	hud.icon_state = "reinforce"
@@ -77,7 +77,7 @@
 	if(assailant)
 		UnregisterSignal(assailant, list(
 			COMSIG_MOVABLE_MOVED,
-			COMSIG_MOVABLE_UPDATE_GLIDE_SIZE,
+			COMSIG_MOVABLE_UPDATED_GLIDE_SIZE,
 		))
 		if(assailant.client)
 			assailant.client.screen -= hud
@@ -86,9 +86,10 @@
 	QDEL_NULL(hud)
 	return ..()
 
-/obj/item/grab/proc/on_update_glide_size(mob/living/grabber, new_size)
+/obj/item/grab/proc/on_updated_glide_size(mob/living/grabber, old_size)
+	SIGNAL_HANDLER  // COMSIG_MOVABLE_UPDATED_GLIDE_SIZE
 	if(affecting && grabber == assailant && affecting != assailant)
-		affecting.set_glide_size(new_size)
+		affecting.set_glide_size(grabber.glide_size)
 
 /obj/item/grab/proc/pull_grabbed(mob/user, turf/old_turf, direct, forced)
 	SIGNAL_HANDLER