Fix FILAMENT_WIDTH_SENSOR measurement

Only measure and store filament width when E is going forward.

Marlin/Marlin.h

   extern bool filament_sensor;  //indicates that filament sensor readings should control extrusion
   extern float filament_width_meas; //holds the filament diameter as accurately measured
   extern int8_t measurement_delay[];  //ring buffer to delay measurement
-  extern int delay_index1, delay_index2;  //ring buffer index. used by planner, temperature, and main code
-  extern float delay_dist; //delay distance counter
+  extern int filwidth_delay_index1, filwidth_delay_index2;  //ring buffer index. used by planner, temperature, and main code
   extern int meas_delay_cm; //delay distance
 #endif
 

Marlin/Marlin_main.cpp

   bool filament_sensor = false;  //M405 turns on filament_sensor control, M406 turns it off
   float filament_width_meas = DEFAULT_MEASURED_FILAMENT_DIA; //Stores the measured filament diameter
   int8_t measurement_delay[MAX_MEASUREMENT_DELAY + 1]; //ring buffer to delay measurement  store extruder factor after subtracting 100
-  int delay_index1 = 0;  //index into ring buffer
-  int delay_index2 = -1;  //index into ring buffer - set to -1 on startup to indicate ring buffer needs to be initialized
-  float delay_dist = 0; //delay distance counter
+  int filwidth_delay_index1 = 0;  //index into ring buffer
+  int filwidth_delay_index2 = -1;  //index into ring buffer - set to -1 on startup to indicate ring buffer needs to be initialized
   int meas_delay_cm = MEASUREMENT_DELAY_CM;  //distance delay setting
 #endif
 
     if (code_seen('D')) meas_delay_cm = code_value();
     NOMORE(meas_delay_cm, MAX_MEASUREMENT_DELAY);
 
-    if (delay_index2 == -1) { //initialize the ring buffer if it has not been done since startup
+    if (filwidth_delay_index2 == -1) { // Initialize the ring buffer if not done since startup
       int temp_ratio = widthFil_to_size_ratio();
 
-      for (delay_index1 = 0; delay_index1 < (int)COUNT(measurement_delay); ++delay_index1)
-        measurement_delay[delay_index1] = temp_ratio - 100;  //subtract 100 to scale within a signed byte
+      for (uint8_t i = 0; i < COUNT(measurement_delay); ++i)
+        measurement_delay[i] = temp_ratio - 100;  // Subtract 100 to scale within a signed byte
 
-      delay_index1 = delay_index2 = 0;
+      filwidth_delay_index1 = filwidth_delay_index2 = 0;
     }
 
     filament_sensor = true;

Marlin/planner.cpp

   block->nominal_rate = ceil(block->step_event_count * inverse_second); // (step/sec) Always > 0
 
   #if ENABLED(FILAMENT_WIDTH_SENSOR)
+    static float filwidth_e_count = 0, filwidth_delay_dist = 0;
+
     //FMM update ring buffer used for delay with filament measurements
-    if (extruder == FILAMENT_SENSOR_EXTRUDER_NUM && delay_index2 > -1) {  //only for extruder with filament sensor and if ring buffer is initialized
+    if (extruder == FILAMENT_SENSOR_EXTRUDER_NUM && filwidth_delay_index2 >= 0) {  //only for extruder with filament sensor and if ring buffer is initialized
+
+      const int MMD_CM = MAX_MEASUREMENT_DELAY + 1, MMD_MM = MMD_CM * 10;
+
+      // increment counters with next move in e axis
+      filwidth_e_count += delta_mm[E_AXIS];
+      filwidth_delay_dist += delta_mm[E_AXIS];
 
-      const int MMD = MAX_MEASUREMENT_DELAY + 1, MMD10 = MMD * 10;
+      // Only get new measurements on forward E movement
+      if (filwidth_e_count > 0.0001) {
 
-      delay_dist += delta_mm[E_AXIS];  // increment counter with next move in e axis
-      while (delay_dist >= MMD10) delay_dist -= MMD10; // loop around the buffer
-      while (delay_dist < 0) delay_dist += MMD10;
+        // Loop the delay distance counter (modulus by the mm length)
+        while (filwidth_delay_dist >= MMD_MM) filwidth_delay_dist -= MMD_MM;
 
-      delay_index1 = delay_dist / 10.0;  // calculate index
-      delay_index1 = constrain(delay_index1, 0, MAX_MEASUREMENT_DELAY); // (already constrained above)
+        // Convert into an index into the measurement array
+        filwidth_delay_index1 = (int)(filwidth_delay_dist / 10.0 + 0.0001);
 
-      if (delay_index1 != delay_index2) { // moved index
-        int8_t meas_sample = widthFil_to_size_ratio() - 100;  // Subtract 100 to reduce magnitude - to store in a signed char
-        while (delay_index1 != delay_index2) {
-          // Increment and loop around buffer
-          if (++delay_index2 >= MMD) delay_index2 -= MMD;
-          delay_index2 = constrain(delay_index2, 0, MAX_MEASUREMENT_DELAY);
-          measurement_delay[delay_index2] = meas_sample;
+        // If the index has changed (must have gone forward)...
+        if (filwidth_delay_index1 != filwidth_delay_index2) {
+          filwidth_e_count = 0; // Reset the E movement counter
+          int8_t meas_sample = widthFil_to_size_ratio() - 100; // Subtract 100 to reduce magnitude - to store in a signed char
+          do {
+            filwidth_delay_index2 = (filwidth_delay_index2 + 1) % MMD_CM; // The next unused slot
+            measurement_delay[filwidth_delay_index2] = meas_sample;       // Store the measurement
+          } while (filwidth_delay_index1 != filwidth_delay_index2);       // More slots to fill?
         }
       }
     }

Marlin/temperature.cpp

   // Control the extruder rate based on the width sensor
   #if ENABLED(FILAMENT_WIDTH_SENSOR)
     if (filament_sensor) {
-      meas_shift_index = delay_index1 - meas_delay_cm;
+      meas_shift_index = filwidth_delay_index1 - meas_delay_cm;
       if (meas_shift_index < 0) meas_shift_index += MAX_MEASUREMENT_DELAY + 1;  //loop around buffer if needed
 
       // Get the delayed info and add 100 to reconstitute to a percent of