Perform the extra purge on M600 if a cooldown happened

Not doing the extra purge is sensible if the swap is performed fast
enough, but if an extruder cooldown happened while waiting for the user
in M600 this is no longer true and the tip might have softened.

Do an extra purge in such cases.

Add some comments and rename UnloadType::User into ::Purge to make the
distinction between the various modes clear.

Firmware/Marlin.h

@@ -490,7 +490,7 @@ void proc_commands();
 
 void M600_load_filament();
 void M600_load_filament_movements();
-void M600_wait_for_user(float HotendTempBckp);
+bool M600_wait_for_user(float HotendTempBckp);
 void M600_check_state(float nozzle_temp);
 void load_filament_final_feed();
 void marlin_wait_for_click();

Firmware/Marlin_main.cpp

@@ -3749,6 +3749,7 @@ static void gcode_M600(bool automatic, bool runout, float x_position, float y_po
     int feedmultiplyBckp = feedmultiply;
     float HotendTempBckp = degTargetHotend(active_extruder);
     int fanSpeedBckp = fanSpeed;
+    bool cooldown = false;
 
     lastpos[X_AXIS] = current_position[X_AXIS];
     lastpos[Y_AXIS] = current_position[Y_AXIS];
@@ -3771,8 +3772,11 @@ static void gcode_M600(bool automatic, bool runout, float x_position, float y_po
     plan_buffer_line_curposXYZE(FILAMENTCHANGE_XYFEED);
     st_synchronize();
 
-    //Beep, manage nozzle heater and wait for user to start unload filament
-    if(!mmu_enabled) M600_wait_for_user(HotendTempBckp);
+    if(!mmu_enabled)
+    {
+        //Beep, manage nozzle heater and wait for user to start unload filament
+        cooldown = M600_wait_for_user(HotendTempBckp);
+    }
 
     lcd_change_fil_state = 0;
 
@@ -3785,7 +3789,9 @@ static void gcode_M600(bool automatic, bool runout, float x_position, float y_po
     else
     {
         // unload filament for single material (used also in M702)
-        unload_filament(runout? UnloadType::Runout: UnloadType::Swap);
+        unload_filament(runout? UnloadType::Runout:
+                        cooldown? UnloadType::Purge:
+                        UnloadType::Swap);
     }
 
     //finish moves
@@ -11984,7 +11990,8 @@ void M600_check_state(float nozzle_temp)
 //! If times out, active extruder temperature is set to 0.
 //!
 //! @param HotendTempBckp Temperature to be restored for active extruder, after user resolves MMU problem.
-void M600_wait_for_user(float HotendTempBckp) {
+//! @return True if the wait involved a hotend cooldown due to timeout.
+bool M600_wait_for_user(float HotendTempBckp) {
 
 		KEEPALIVE_STATE(PAUSED_FOR_USER);
 
@@ -11993,6 +12000,7 @@ void M600_wait_for_user(float HotendTempBckp) {
 		uint8_t wait_for_user_state = 0;
 		lcd_display_message_fullscreen_P(_T(MSG_PRESS_TO_UNLOAD));
 		bool bFirst=true;
+		bool bCooldown=false;
 
 		while (!(wait_for_user_state == 0 && lcd_clicked())){
 			manage_heater();
@@ -12025,6 +12033,7 @@ void M600_wait_for_user(float HotendTempBckp) {
 					lcd_display_message_fullscreen_P(_i("Press the knob to preheat nozzle and continue."));////MSG_PRESS_TO_PREHEAT c=20 r=4
 					wait_for_user_state = 1;
 					setAllTargetHotends(0);
+					bCooldown = true;
 					st_synchronize();
 					disable_e0();
 					disable_e1();
@@ -12059,6 +12068,7 @@ void M600_wait_for_user(float HotendTempBckp) {
 
 		}
 		WRITE(BEEPER, LOW);
+		return bCooldown;
 }
 
 void M600_load_filament_movements()

Firmware/ultralcd.cpp

@@ -6288,7 +6288,7 @@ void unload_filament(UnloadType unload)
 	lcd_setstatuspgm(_T(MSG_UNLOADING_FILAMENT));
 
     raise_z_above(unload == UnloadType::Swap? MIN_Z_FOR_SWAP: MIN_Z_FOR_UNLOAD);
-    if (unload == UnloadType::User)
+    if (unload == UnloadType::Purge)
     {
         // extrude slowly
         current_position[E_AXIS] += FILAMENTCHANGE_UNLOADFEED;

Firmware/ultralcd.h

@@ -196,12 +196,12 @@ void lcd_generic_preheat_menu();
 
 enum class UnloadType : uint8_t
 {
-    User,      // user-triggered unload
-    Swap,      // part of a M600 sequence
-    Runout,    // triggered by runout
+    Purge,     // user-triggered unload, perform an extra purge before unload
+    Swap,      // part of an M600 sequence (no extra purge necessary)
+    Runout,    // triggered by runout (extra purge not possible)
 };
 
-void unload_filament(UnloadType unload=UnloadType::User);
+void unload_filament(UnloadType unload=UnloadType::Purge);
 
 
 void lcd_printer_connected();