Tweak stepper shutdown test

Marlin/src/MarlinCore.cpp

   const millis_t ms = millis();
 
   // Prevent steppers timing-out in the middle of M600
-  #define STAY_TEST (BOTH(ADVANCED_PAUSE_FEATURE, PAUSE_PARK_NO_STEPPER_TIMEOUT) && did_pause_print)
+  // unless PAUSE_PARK_NO_STEPPER_TIMEOUT is disabled
+  const bool parked_or_ignoring = ignore_stepper_queue ||
+     (BOTH(ADVANCED_PAUSE_FEATURE, PAUSE_PARK_NO_STEPPER_TIMEOUT) && did_pause_print);
 
-  if (STAY_TEST || ignore_stepper_queue)
-    gcode.reset_stepper_timeout(ms);
+  // Reset both the M18/M84 activity timeout and the M85 max 'kill' timeout
+  if (parked_or_ignoring) gcode.reset_stepper_timeout(ms);
 
   if (gcode.stepper_max_timed_out(ms)) {
     SERIAL_ERROR_START();
     kill();
   }
 
+  // M18 / M94 : Handle steppers inactive time timeout
   if (gcode.stepper_inactive_time) {
+
     static bool already_shutdown_steppers; // = false
+
+    // Any moves in the planner? Resets both the M18/M84
+    // activity timeout and the M85 max 'kill' timeout
     if (planner.has_blocks_queued())
       gcode.reset_stepper_timeout(ms);
-    else if (!STAY_TEST && !ignore_stepper_queue && gcode.stepper_inactive_timeout()) {
+    else if (!parked_or_ignoring && gcode.stepper_inactive_timeout()) {
       if (!already_shutdown_steppers) {
         already_shutdown_steppers = true;  // L6470 SPI will consume 99% of free time without this
+
+        // Individual axes will be disabled if configured
         if (ENABLED(DISABLE_INACTIVE_X)) DISABLE_AXIS_X();
         if (ENABLED(DISABLE_INACTIVE_Y)) DISABLE_AXIS_Y();
         if (ENABLED(DISABLE_INACTIVE_Z)) DISABLE_AXIS_Z();
         if (ENABLED(DISABLE_INACTIVE_E)) disable_e_steppers();
+
         #if BOTH(HAS_LCD_MENU, AUTO_BED_LEVELING_UBL)
           if (ubl.lcd_map_control) {
             ubl.lcd_map_control = false;