Only watch the heater that was set

Marlin/Marlin_main.cpp

@@ -3168,7 +3168,7 @@ inline void gcode_M104() {
     #endif
 
     #ifdef WATCH_TEMP_PERIOD
-      start_watching_heaters();
+      start_watching_heater(target_extruder);
     #endif
   }
 }
@@ -3282,7 +3282,7 @@ inline void gcode_M109() {
   #endif
 
   #ifdef WATCH_TEMP_PERIOD
-    start_watching_heaters();
+    start_watching_heater(target_extruder);
   #endif
 
   millis_t temp_ms = millis();

Marlin/temperature.cpp

@@ -1005,9 +1005,8 @@ void tp_init() {
    * their target temperature by a configurable margin.
    * This is called when the temperature is set. (M104, M109)
    */
-  void start_watching_heaters() {
+  void start_watching_heater(int e) {
     millis_t ms = millis() + WATCH_TEMP_PERIOD;
-      for (int e = 0; e < EXTRUDERS; e++) {
     if (degHotend(e) < degTargetHotend(e) - (WATCH_TEMP_INCREASE * 2)) {
       watch_target_temp[e] = degHotend(e) + WATCH_TEMP_INCREASE;
       watch_heater_next_ms[e] = ms;
@@ -1015,7 +1014,6 @@ void tp_init() {
     else
       watch_heater_next_ms[e] = 0;
   }
-  }
 #endif
 
 #if HAS_HEATER_THERMAL_PROTECTION || HAS_BED_THERMAL_PROTECTION

Marlin/temperature.h

@@ -138,7 +138,7 @@ void setExtruderAutoFanState(int pin, bool state);
 void checkExtruderAutoFans();
 
 #ifdef WATCH_TEMP_PERIOD
-  void start_watching_heaters();
+  void start_watching_heater(int e=0);
 #endif
 
 FORCE_INLINE void autotempShutdown() {

Marlin/ultralcd.cpp

@@ -522,7 +522,7 @@ void _lcd_preheat(int endnum, const float temph, const float tempb, const int fa
   fanSpeed = fan;
   lcd_return_to_status();
   #ifdef WATCH_TEMP_PERIOD
-    start_watching_heaters();
+    if (endnum >= 0) start_watching_heater(endnum);
   #endif
 }
 void lcd_preheat_pla0() { _lcd_preheat(0, plaPreheatHotendTemp, plaPreheatHPBTemp, plaPreheatFanSpeed); }