Merge pull request MarlinFirmware#4162 from leptun/fix_lcd_encoder_diff

Fix `lcd_encoder_diff` getting out of sync with the knob hard steps

Firmware/lcd.cpp

@@ -4,6 +4,7 @@
 #include <stdio.h>
 #include <stdarg.h>
 #include <avr/pgmspace.h>
+#include <util/atomic.h>
 #include <util/delay.h>
 #include "Timer.h"
 
@@ -684,13 +685,20 @@ void lcd_knob_update() {
 	if (lcd_backlight_wake_trigger) {
 		lcd_backlight_wake_trigger = false;
 		backlight_wake();
-		if (abs(lcd_encoder_diff) >= ENCODER_PULSES_PER_STEP) {
-			lcd_encoder += lcd_encoder_diff / ENCODER_PULSES_PER_STEP;
-			lcd_encoder_diff = 0;
-			Sound_MakeSound(e_SOUND_TYPE_EncoderMove);
-		} else {
-			Sound_MakeSound(e_SOUND_TYPE_ButtonEcho);
+		bool did_rotate = false;
+		ATOMIC_BLOCK(ATOMIC_RESTORESTATE) {
+			if (abs(lcd_encoder_diff) >= ENCODER_PULSES_PER_STEP) {
+				lcd_encoder += lcd_encoder_diff / ENCODER_PULSES_PER_STEP;
+				lcd_encoder_diff %= ENCODER_PULSES_PER_STEP;
+				did_rotate = true;
+			}
+			else {
+				// Get lcd_encoder_diff in sync with the encoder hard steps.
+				// We assume that a click happens only when the knob is rotated into a stable position
+				lcd_encoder_diff = 0;
+			}
 		}
+		Sound_MakeSound(did_rotate ? e_SOUND_TYPE_EncoderMove : e_SOUND_TYPE_ButtonEcho);
 
 		if (lcd_draw_update == 0) {
 			// Update LCD rendering at minimum