Fix logic of UBL::fade_scaling_factor_for_z

Marlin/Marlin_main.cpp

   /**
    * Prepare a linear move in a Cartesian setup.
    * If Mesh Bed Leveling is enabled, perform a mesh move.
+   *
+   * Returns true if the caller didn't update current_position.
    */
   inline bool prepare_move_to_destination_cartesian() {
     // Do not use feedrate_percentage for E or Z only moves
         else
       #elif ENABLED(AUTO_BED_LEVELING_UBL)
         if (ubl.state.active) {
-
           ubl_line_to_destination(MMS_SCALED(feedrate_mm_s), active_extruder);
-
           return false;
         }
         else

Marlin/UBL.h

         float g29_correction_fade_height = 10.0,
               g29_fade_height_multiplier = 1.0 / 10.0; // It's cheaper to do a floating point multiply than divide,
                                                        // so keep this value and its reciprocal.
-      #else
-        const float g29_correction_fade_height = 10.0,
-                    g29_fade_height_multiplier = 1.0 / 10.0;
       #endif
 
       // If you change this struct, adjust TOTAL_STRUCT_SIZE
     class unified_bed_leveling {
       private:
 
-        static float last_specified_z,
-                     fade_scaling_factor_for_current_height;
+        static float last_specified_z;
 
       public:
 
         }
 
         /**
-         * This routine is used to scale the Z correction depending upon the current nozzle height. It is
-         * optimized for speed. It avoids floating point operations by checking if the requested scaling
-         * factor is going to be the same as the last time the function calculated a value. If so, it just
-         * returns it.
+         * This function sets the Z leveling fade factor based on the given Z height,
+         * only re-calculating when necessary.
          *
-         * It returns a scaling factor of 1.0 if UBL is inactive.
-         * It returns a scaling factor of 0.0 if Z is past the specified 'Fade Height'
+         *  Returns 1.0 if g29_correction_fade_height is 0.0.
+         *  Returns 0.0 if Z is past the specified 'Fade Height'.
          */
         #if ENABLED(ENABLE_LEVELING_FADE_HEIGHT)
 
           static FORCE_INLINE float fade_scaling_factor_for_z(const float &lz) {
+            if (state.g29_correction_fade_height == 0.0) return 1.0;
+
+            static float fade_scaling_factor = 1.0;
             const float rz = RAW_Z_POSITION(lz);
             if (last_specified_z != rz) {
               last_specified_z = rz;
-              fade_scaling_factor_for_current_height =
-                state.active && rz < state.g29_correction_fade_height
+              fade_scaling_factor =
+                rz < state.g29_correction_fade_height
                   ? 1.0 - (rz * state.g29_fade_height_multiplier)
                   : 0.0;
             }
-            return fade_scaling_factor_for_current_height;
+            return fade_scaling_factor;
           }
 
-        #else
-
-          static constexpr float fade_scaling_factor_for_z(const float &lz) { UNUSED(lz); return 1.0; }
-
         #endif
 
     }; // class unified_bed_leveling

Marlin/UBL_Bed_Leveling.cpp

 
   float unified_bed_leveling::z_values[UBL_MESH_NUM_X_POINTS][UBL_MESH_NUM_Y_POINTS],
         unified_bed_leveling::last_specified_z,
-        unified_bed_leveling::fade_scaling_factor_for_current_height,
         unified_bed_leveling::mesh_index_to_xpos[UBL_MESH_NUM_X_POINTS + 1], // +1 safety margin for now, until determinism prevails
         unified_bed_leveling::mesh_index_to_ypos[UBL_MESH_NUM_Y_POINTS + 1];
 
        * updated, but until then, we try to ease the transition
        * for our Beta testers.
        */
-      if (ubl.state.g29_fade_height_multiplier != 1.0 / ubl.state.g29_correction_fade_height) {
-        ubl.state.g29_fade_height_multiplier = 1.0 / ubl.state.g29_correction_fade_height;
+      const float recip = ubl.state.g29_correction_fade_height ? 1.0 / ubl.state.g29_correction_fade_height : 1.0;
+      if (ubl.state.g29_fade_height_multiplier != recip) {
+        ubl.state.g29_fade_height_multiplier = recip;
         store_state();
       }
     #endif
     ZERO(z_values);
 
     last_specified_z = -999.9;
-    fade_scaling_factor_for_current_height = 0.0;
   }
 
   void unified_bed_leveling::invalidate() {

Marlin/UBL_G29.cpp

     safe_delay(50);
 
     #if ENABLED(ENABLE_LEVELING_FADE_HEIGHT)
-      SERIAL_PROTOCOLPAIR("g29_correction_fade_height : ", ubl.state.g29_correction_fade_height);
-      SERIAL_EOL;
+      SERIAL_PROTOCOLLNPAIR("g29_correction_fade_height : ", ubl.state.g29_correction_fade_height);
     #endif
 
     SERIAL_PROTOCOLPGM("z_offset: ");