Clear LCD button state, apply timer to all

Addressing #3007

Marlin/ultralcd.cpp

@@ -2361,6 +2361,7 @@ void kill_screen(const char* lcd_msg) {
 
   void lcd_quick_feedback() {
     lcdDrawUpdate = LCDVIEW_CLEAR_CALL_REDRAW;
+    buttons = 0;
     next_button_update_ms = millis() + 500;
 
     // Buzz and wait. The delay is needed for buttons to settle!
@@ -2850,20 +2851,26 @@ void lcd_reset_alert_level() { lcd_status_message_level = 0; }
    * Warning: This function is called from interrupt context!
    */
   void lcd_buttons_update() {
+    millis_t now = millis();
+    if (ELAPSED(now, next_button_update_ms)) {
+
       #if ENABLED(NEWPANEL)
         uint8_t newbutton = 0;
+
         #if BUTTON_EXISTS(EN1)
           if (BUTTON_PRESSED(EN1)) newbutton |= EN_A;
         #endif
+
         #if BUTTON_EXISTS(EN2)
           if (BUTTON_PRESSED(EN2)) newbutton |= EN_B;
         #endif
-      #if LCD_HAS_DIRECTIONAL_BUTTONS || BUTTON_EXISTS(ENC)
-        millis_t now = millis();
+
+        #if BUTTON_EXISTS(ENC)
+          if (BUTTON_PRESSED(ENC)) newbutton |= EN_C;
         #endif
 
         #if LCD_HAS_DIRECTIONAL_BUTTONS
-        if (ELAPSED(now, next_button_update_ms)) {
+
           if (false) {
             // for the else-ifs below
           }
@@ -2891,12 +2898,8 @@ void lcd_reset_alert_level() { lcd_status_message_level = 0; }
               next_button_update_ms = now + 300;
             }
           #endif
-        }
-      #endif
 
-      #if BUTTON_EXISTS(ENC)
-        if (ELAPSED(now, next_button_update_ms) && BUTTON_PRESSED(ENC)) newbutton |= EN_C;
-      #endif
+        #endif // LCD_HAS_DIRECTIONAL_BUTTONS
 
         buttons = newbutton;
         #if ENABLED(LCD_HAS_SLOW_BUTTONS)
@@ -2909,6 +2912,8 @@ void lcd_reset_alert_level() { lcd_status_message_level = 0; }
         GET_BUTTON_STATES(buttons);
       #endif //!NEWPANEL
 
+    } // next_button_update_ms
+
     // Manage encoder rotation
     #if ENABLED(REVERSE_MENU_DIRECTION) && ENABLED(REVERSE_ENCODER_DIRECTION)
       #define ENCODER_DIFF_CW  (encoderDiff -= encoderDirection)