Non-reentrant "Moving..." screen to safely wait in LCD

Marlin/language_en.h

 #ifndef MSG_LEVEL_BED
   #define MSG_LEVEL_BED                       "Level bed"
 #endif
+#ifndef MSG_MOVING
+  #define MSG_MOVING                          "Moving..."
+#endif
 #ifndef MSG_MOVE_X
   #define MSG_MOVE_X                          "Move X"
 #endif

Marlin/ultralcd.cpp

   bool screen_changed;
 
   // LCD and menu clicks
-  bool lcd_clicked, wait_for_unclick, defer_return_to_status;
+  bool lcd_clicked, wait_for_unclick, defer_return_to_status, no_reentrance;
 
   // Variables used when editing values.
   const char* editLabel;
     }
   }
 
+  /**
+   * Synchronize safely while holding the current screen
+   * This blocks all further screen or stripe updates once called
+   */
+  inline void lcd_synchronize() {
+    lcd_implementation_drawmenu_static(LCD_HEIGHT >= 4 ? 1 : 0, PSTR(MSG_MOVING));
+    if (no_reentrance) return;
+    no_reentrance = true;
+    screenFunc_t old_screen = currentScreen;
+    lcd_goto_screen(lcd_synchronize);
+    stepper.synchronize();
+    no_reentrance = false;
+    lcd_goto_screen(old_screen);
+  }
+
+  inline void lcd_wait_for_homing() {
+    no_reentrance = true;
+    while (!axis_homed[X_AXIS] || !axis_homed[Y_AXIS] || !axis_homed[Z_AXIS]) idle();
+    no_reentrance = true;
+  }
+
   void lcd_return_to_status() { lcd_goto_screen(lcd_status_screen); }
 
   void lcd_save_previous_screen() {
     // Note: During Manual Bed Leveling the homed Z position is MESH_HOME_SEARCH_Z
     // Z position will be restored with the final action, a G28
     inline void _mbl_goto_xy(float x, float y) {
+      if (no_reentrance) return;
       current_position[Z_AXIS] = LOGICAL_Z_POSITION(MESH_HOME_SEARCH_Z + Z_HOMING_HEIGHT);
       line_to_current(Z_AXIS);
       current_position[X_AXIS] = LOGICAL_X_POSITION(x);
         current_position[Z_AXIS] = LOGICAL_Z_POSITION(MESH_HOME_SEARCH_Z);
         line_to_current(Z_AXIS);
       #endif
-      stepper.synchronize();
+      lcd_synchronize();
     }
 
     void _lcd_level_goto_next_point();
     void _lcd_level_bed_get_z() {
       ENCODER_DIRECTION_NORMAL();
 
+      if (no_reentrance) goto KeepDrawing;
+
       // Encoder wheel adjusts the Z position
       if (encoderPosition) {
         refresh_cmd_timeout();
 
             current_position[Z_AXIS] = MESH_HOME_SEARCH_Z + Z_HOMING_HEIGHT;
             line_to_current(Z_AXIS);
-            stepper.synchronize();
+            lcd_synchronize();
 
             mbl.set_has_mesh(true);
             enqueue_and_echo_commands_P(PSTR("G28"));
         debounce_click = false;
       }
 
+KeepDrawing:
       // Update on first display, then only on updates to Z position
       // Show message above on clicks instead
       if (lcdDrawUpdate) {
           LCDVIEW_CALL_NO_REDRAW
         #endif
       ;
-      if (axis_homed[X_AXIS] && axis_homed[Y_AXIS] && axis_homed[Z_AXIS])
-        lcd_goto_screen(_lcd_level_bed_homing_done);
+      if (no_reentrance) return;
+      lcd_wait_for_homing();
+      lcd_goto_screen(_lcd_level_bed_homing_done);
     }
 
     /**

Marlin/ultralcd_impl_DOGM.h

     u8g.setPrintPos((START_COL) * (DOG_CHAR_WIDTH), row_y2);
   }
 
-  #if ENABLED(LCD_INFO_MENU) || ENABLED(FILAMENT_CHANGE_FEATURE)
+  // Draw a static line of text in the same idiom as a menu item
+  static void lcd_implementation_drawmenu_static(const uint8_t row, const char* pstr, const bool center=true, const bool invert=false, const char* valstr=NULL) {
 
-    // Draw a static line of text in the same idiom as a menu item
-    static void lcd_implementation_drawmenu_static(const uint8_t row, const char* pstr, const bool center=true, const bool invert=false, const char* valstr=NULL) {
+    lcd_implementation_mark_as_selected(row, invert);
 
-      lcd_implementation_mark_as_selected(row, invert);
-
-      if (!PAGE_CONTAINS(row_y1, row_y2)) return;
+    if (!PAGE_CONTAINS(row_y1, row_y2)) return;
 
-      char c;
-      int8_t n = LCD_WIDTH - (START_COL);
+    char c;
+    int8_t n = LCD_WIDTH - (START_COL);
 
-      if (center && !valstr) {
-        int8_t pad = (LCD_WIDTH - lcd_strlen_P(pstr)) / 2;
-        while (--pad >= 0) { u8g.print(' '); n--; }
-      }
-      while (n > 0 && (c = pgm_read_byte(pstr))) {
-        n -= lcd_print_and_count(c);
-        pstr++;
-      }
-      if (valstr) while (n > 0 && (c = *valstr)) {
-        n -= lcd_print_and_count(c);
-        valstr++;
-      }
-      while (n-- > 0) u8g.print(' ');
+    if (center && !valstr) {
+      int8_t pad = (LCD_WIDTH - lcd_strlen_P(pstr)) / 2;
+      while (--pad >= 0) { u8g.print(' '); n--; }
     }
-
-  #endif // LCD_INFO_MENU || FILAMENT_CHANGE_FEATURE
+    while (n > 0 && (c = pgm_read_byte(pstr))) {
+      n -= lcd_print_and_count(c);
+      pstr++;
+    }
+    if (valstr) while (n > 0 && (c = *valstr)) {
+      n -= lcd_print_and_count(c);
+      valstr++;
+    }
+    while (n-- > 0) u8g.print(' ');
+  }
 
   // Draw a generic menu item
   static void lcd_implementation_drawmenu_generic(const bool isSelected, const uint8_t row, const char* pstr, const char pre_char, const char post_char) {

Marlin/ultralcd_impl_HD44780.h

 
 #if ENABLED(ULTIPANEL)
 
-  #if ENABLED(LCD_INFO_MENU) || ENABLED(FILAMENT_CHANGE_FEATURE)
-
-    static void lcd_implementation_drawmenu_static(const uint8_t row, const char* pstr, const bool center=true, const bool invert=false, const char *valstr=NULL) {
-      UNUSED(invert);
-      char c;
-      int8_t n = LCD_WIDTH;
-      lcd.setCursor(0, row);
-      if (center && !valstr) {
-        int8_t pad = (LCD_WIDTH - lcd_strlen_P(pstr)) / 2;
-        while (--pad >= 0) { lcd.print(' '); n--; }
-      }
-      while (n > 0 && (c = pgm_read_byte(pstr))) {
-        n -= charset_mapper(c);
-        pstr++;
-      }
-      if (valstr) while (n > 0 && (c = *valstr)) {
-        n -= charset_mapper(c);
-        valstr++;
-      }
-      while (n-- > 0) lcd.print(' ');
+  static void lcd_implementation_drawmenu_static(const uint8_t row, const char* pstr, const bool center=true, const bool invert=false, const char *valstr=NULL) {
+    UNUSED(invert);
+    char c;
+    int8_t n = LCD_WIDTH;
+    lcd.setCursor(0, row);
+    if (center && !valstr) {
+      int8_t pad = (LCD_WIDTH - lcd_strlen_P(pstr)) / 2;
+      while (--pad >= 0) { lcd.print(' '); n--; }
     }
-
-  #endif // LCD_INFO_MENU || FILAMENT_CHANGE_FEATURE
+    while (n > 0 && (c = pgm_read_byte(pstr))) {
+      n -= charset_mapper(c);
+      pstr++;
+    }
+    if (valstr) while (n > 0 && (c = *valstr)) {
+      n -= charset_mapper(c);
+      valstr++;
+    }
+    while (n-- > 0) lcd.print(' ');
+  }
 
   static void lcd_implementation_drawmenu_generic(const bool sel, const uint8_t row, const char* pstr, const char pre_char, const char post_char) {
     char c;