Chemmaster and chemdispenser UI disables if the machine is not operable (#20101)

Chemmaster and chemdispenser UI disables if the machine is not operable.
Turned a comment into a DMDoc.

Fixes #19825

code/modules/power/power_usage.dm

@@ -28,10 +28,13 @@ This is /obj/machinery level code to properly manage power usage from the area.
 		chan = power_channel
 	return check_area.powered(chan)			// return power status of the area
 
-// called whenever the power settings of the containing area change
-// by default, check equipment channel & set flag can override if needed
-// This is NOT for when the machine's own status changes; update_use_power for that.
+/// called whenever the power settings of the containing area change
+/// by default, check equipment channel & set flag can override if needed
+/// This is NOT for when the machine's own status changes; update_use_power for that.
 /obj/machinery/proc/power_change()
+	SHOULD_NOT_SLEEP(TRUE)
+	SHOULD_CALL_PARENT(TRUE)
+
 	var/oldstat = stat
 
 	if(powered(power_channel))

code/modules/reagents/Chemistry-Machinery.dm

@@ -299,6 +299,11 @@
 		return TRUE
 	return TRUE
 
+/obj/machinery/chem_master/ui_status(mob/user, datum/ui_state/state)
+	if(!operable())
+		return UI_DISABLED
+
+	. = ..()
 
 
 /obj/machinery/chem_master/Topic(href, href_list)

code/modules/reagents/dispenser/dispenser2.dm

@@ -202,6 +202,12 @@
 
 	add_fingerprint(usr)
 
+/obj/machinery/chemical_dispenser/ui_status(mob/user, datum/ui_state/state)
+	if(!operable())
+		return UI_DISABLED
+
+	. = ..()
+
 /obj/machinery/chemical_dispenser/attack_ai(mob/user as mob)
 	if(!ai_can_interact(user))
 		return