Add a fade factor for mesh leveling

Marlin/Marlin_main.cpp

@@ -2270,6 +2270,30 @@ static void clean_up_after_endstop_or_probe_move() {
     #endif
   }
 
+  #if ENABLED(ENABLE_LEVELING_FADE_HEIGHT)
+
+    void set_z_fade_height(const float zfh) {
+      planner.z_fade_height = zfh;
+      planner.inverse_z_fade_height = RECIPROCAL(zfh);
+
+      if (
+        #if ENABLED(MESH_BED_LEVELING)
+          mbl.active()
+        #else
+          planner.abl_enabled
+        #endif
+      ) {
+        set_current_from_steppers_for_axis(
+          #if ABL_PLANAR
+            ALL_AXES
+          #else
+            Z_AXIS
+          #endif
+        );
+      }
+    }
+
+  #endif // LEVELING_FADE_HEIGHT
 
   /**
    * Reset calibration results to zero.
@@ -6788,9 +6812,17 @@ void quickstop_stepper() {
 
 #if PLANNER_LEVELING
   /**
-   * M420: Enable/Disable Bed Leveling
+   * M420: Enable/Disable Bed Leveling and/or set the Z fade height.
+   *
+   *       S[bool]   Turns leveling on or off
+   *       Z[height] Sets the Z fade height (0 or none to disable)
    */
-  inline void gcode_M420() { if (code_seen('S')) set_bed_leveling_enabled(code_value_bool()); }
+  inline void gcode_M420() {
+    if (code_seen('S')) set_bed_leveling_enabled(code_value_bool());
+    #if ENABLED(ENABLE_LEVELING_FADE_HEIGHT)
+      if (code_seen('Z')) set_z_fade_height(code_value_linear_units());
+    #endif
+  }
 #endif
 
 #if ENABLED(MESH_BED_LEVELING)