Fix menu highlight glitch, tweak scrolling code (#9957)

Marlin/src/lcd/ultralcd.cpp

 uint16_t max_display_update_time = 0;
 
 #if ENABLED(DOGLCD)
-  bool drawing_screen = false;
+  bool drawing_screen, // = false
+       first_page;
+#else
+  constexpr bool first_page = true;
 #endif
 
 #if ENABLED(DAC_STEPPER_CURRENT)
   #endif
 
   bool no_reentry = false;
+  constexpr int8_t menu_bottom = LCD_HEIGHT - (TALL_FONT_CORRECTION);
 
   ////////////////////////////////////////////
   ///////////////// Menu Tree ////////////////
    *   _lcdLineNr is the index of the LCD line (e.g., 0-3)
    *   _menuLineNr is the menu item to draw and process
    *   _thisItemNr is the index of each MENU_ITEM or STATIC_ITEM
-   *   _countedItems is the total number of items in the menu (after one call)
+   *   screen_items is the total number of items in the menu (after one call)
    */
   #define START_SCREEN_OR_MENU(LIMIT) \
     ENCODER_DIRECTION_MENUS(); \
     ENCODER_RATE_MULTIPLY(false); \
     if (encoderPosition > 0x8000) encoderPosition = 0; \
-    static int8_t _countedItems = 0; \
-    int8_t encoderLine = encoderPosition / (ENCODER_STEPS_PER_MENU_ITEM); \
-    if (_countedItems > 0 && encoderLine >= _countedItems - (LIMIT)) { \
-      encoderLine = max(0, _countedItems - (LIMIT)); \
+    if (first_page) encoderLine = encoderPosition / (ENCODER_STEPS_PER_MENU_ITEM); \
+    if (screen_items > 0 && encoderLine >= screen_items - (LIMIT)) { \
+      encoderLine = max(0, screen_items - (LIMIT)); \
       encoderPosition = encoderLine * (ENCODER_STEPS_PER_MENU_ITEM); \
     }
 
   #define SCREEN_OR_MENU_LOOP() \
     int8_t _menuLineNr = encoderTopLine, _thisItemNr; \
-    for (int8_t _lcdLineNr = 0; _lcdLineNr < LCD_HEIGHT - (TALL_FONT_CORRECTION); _lcdLineNr++, _menuLineNr++) { \
+    for (int8_t _lcdLineNr = 0; _lcdLineNr < menu_bottom; _lcdLineNr++, _menuLineNr++) { \
       _thisItemNr = 0
 
   /**
    *               Scroll as-needed to keep the selected line in view.
    */
   #define START_SCREEN() \
-    START_SCREEN_OR_MENU(LCD_HEIGHT - (TALL_FONT_CORRECTION)); \
-    encoderTopLine = encoderLine; \
+    scroll_screen(menu_bottom, false); \
     bool _skipStatic = false; \
     SCREEN_OR_MENU_LOOP()
 
   #define START_MENU() \
-    START_SCREEN_OR_MENU(1); \
-    screen_changed = false; \
-    NOMORE(encoderTopLine, encoderLine); \
-    if (encoderLine >= encoderTopLine + LCD_HEIGHT - (TALL_FONT_CORRECTION)) { \
-      encoderTopLine = encoderLine - (LCD_HEIGHT - (TALL_FONT_CORRECTION) - 1); \
-    } \
+    scroll_screen(1, true); \
     bool _skipStatic = true; \
     SCREEN_OR_MENU_LOOP()
 
   #define END_SCREEN() \
     } \
-    _countedItems = _thisItemNr
+    screen_items = _thisItemNr
 
   #define END_MENU() \
     } \
-    _countedItems = _thisItemNr; \
+    screen_items = _thisItemNr; \
     UNUSED(_skipStatic)
 
   ////////////////////////////////////////////
     lcd_goto_previous_menu();
   }
 
+  /**
+   * Scrolling for menus and other line-based screens
+   */
+  int8_t encoderLine, screen_items;
+  void scroll_screen(const uint8_t limit, const bool is_menu) {
+    if (encoderPosition > 0x8000) encoderPosition = 0;
+    if (first_page) {
+      encoderLine = encoderPosition / (ENCODER_STEPS_PER_MENU_ITEM);
+      screen_changed = false;
+    }
+    if (screen_items > 0 && encoderLine >= screen_items - limit) {
+      encoderLine = max(0, screen_items - limit);
+      encoderPosition = encoderLine * (ENCODER_STEPS_PER_MENU_ITEM);
+    }
+    if (is_menu) {
+      NOMORE(encoderTopLine, encoderLine);
+      if (encoderLine >= encoderTopLine + menu_bottom)
+        encoderTopLine = encoderLine - menu_bottom + 1;
+    }
+    else
+      encoderTopLine = encoderLine;
+  }
+
 #endif // ULTIPANEL
 
 /**
 
   void lcd_quick_feedback(const bool clear_buttons) {
     lcdDrawUpdate = LCDVIEW_CLEAR_CALL_REDRAW;
-
-    if (clear_buttons)
-      buttons = 0;
-
+    if (clear_buttons) buttons = 0;
     next_button_update_ms = millis() + 500;
 
     // Buzz and wait. The delay is needed for buttons to settle!
     #endif
 
     #if HAS_LCD_CONTRAST
-      // please don't remove the "(int16_t*)" - it's needed for the VIKI2 display  --- see PR #9132 before changing it
       MENU_ITEM_EDIT_CALLBACK(int3, MSG_CONTRAST, &lcd_contrast, LCD_CONTRAST_MIN, LCD_CONTRAST_MAX, lcd_callback_set_contrast, true);
     #endif
     #if ENABLED(FWRETRACT)
         if (do_u8g_loop) {
           if (!drawing_screen) {                        // If not already drawing pages
             u8g.firstPage();                            // Start the first page
-            drawing_screen = 1;                         // Flag as drawing pages
+            drawing_screen = first_page = true;         // Flag as drawing pages
           }
           lcd_setFont(FONT_MENU);                       // Setup font for every page draw
           u8g.setColorIndex(1);                         // And reset the color
           CURRENTSCREEN();                              // Draw and process the current screen
+          first_page = false;
 
           // The screen handler can clear drawing_screen for an action that changes the screen.
           // If still drawing and there's another page, update max-time and return now.
         #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 BUTTON_EXISTS(ENC)
           if (BUTTON_PRESSED(ENC)) newbutton |= EN_C;
         #endif