Fix incorrect UTF filename scrolling

Marlin/src/lcd/dogm/marlinui_DOGM.cpp

@@ -461,8 +461,11 @@ void MarlinUI::clear_lcd() { } // Automatically cleared by Picture Loop
 
     void MenuItem_sdbase::draw(const bool sel, const uint8_t row, PGM_P const, CardReader &theCard, const bool isDir) {
       if (mark_as_selected(row, sel)) {
-        if (isDir) lcd_put_wchar(LCD_STR_FOLDER[0]);
+        uint8_t maxlen = LCD_WIDTH;
-        constexpr uint8_t maxlen = LCD_WIDTH - 1;
+        if (isDir) {
+          lcd_put_wchar(LCD_STR_FOLDER[0]);
+          maxlen--;
+        }
         const pixel_len_t pixw = maxlen * (MENU_FONT_WIDTH);
         pixel_len_t n = pixw - lcd_put_u8str_max(ui.scrolled_filename(theCard, maxlen, row, sel), pixw);
         while (n > MENU_FONT_WIDTH) n -= lcd_put_wchar(' ');

Marlin/src/lcd/fontutils.cpp

@@ -75,6 +75,11 @@ int pf_bsearch_r(void *userdata, size_t num_data, pf_bsearch_cb_comp_t cb_comp,
   return -1;
 }
 
+/* Returns true if passed byte is first byte of UTF-8 char sequence */
+static inline bool utf8_is_start_byte_of_char(const uint8_t b) {
+  return 0x80 != (b & 0xC0);
+}
+
 /* This function gets the character at the pstart position, interpreting UTF8 multibyte sequences
    and returns the pointer to the next character */
 uint8_t* get_utf8_value_cb(uint8_t *pstart, read_byte_cb_t cb_read_byte, wchar_t *pval) {
@@ -131,8 +136,8 @@ uint8_t* get_utf8_value_cb(uint8_t *pstart, read_byte_cb_t cb_read_byte, wchar_t
       p++;
     }
   #endif
-  else if (0x80 == (0xC0 & valcur))
+  else if (!utf8_is_start_byte_of_char(valcur))
-    for (; 0x80 == (0xC0 & valcur); ) { p++; valcur = cb_read_byte(p); }
+    for (; !utf8_is_start_byte_of_char(valcur); ) { p++; valcur = cb_read_byte(p); }
   else
     for (; 0xFC < (0xFE & valcur); ) { p++; valcur = cb_read_byte(p); }
 
@@ -143,12 +148,12 @@ uint8_t* get_utf8_value_cb(uint8_t *pstart, read_byte_cb_t cb_read_byte, wchar_t
 
 static inline uint8_t utf8_strlen_cb(const char *pstart, read_byte_cb_t cb_read_byte) {
   uint8_t cnt = 0;
-  uint8_t *pnext = (uint8_t *)pstart;
+  uint8_t *p = (uint8_t *)pstart;
   for (;;) {
-    wchar_t ch;
+    uint8_t b = cb_read_byte(p);
-    pnext = get_utf8_value_cb(pnext, cb_read_byte, &ch);
+    if (!b) break;
-    if (!ch) break;
+    if (utf8_is_start_byte_of_char(b)) cnt++;
-    cnt++;
+    p++;
   }
   return cnt;
 }
@@ -160,3 +165,26 @@ uint8_t utf8_strlen(const char *pstart) {
 uint8_t utf8_strlen_P(PGM_P pstart) {
   return utf8_strlen_cb(pstart, read_byte_rom);
 }
+
+static inline uint8_t utf8_byte_pos_by_char_num_cb(const char *pstart, read_byte_cb_t cb_read_byte, const uint8_t charnum) {
+  uint8_t *p = (uint8_t *)pstart;
+  uint8_t char_idx = 0;
+  uint8_t byte_idx = 0;
+  for (;;) {
+    uint8_t b = cb_read_byte(p+byte_idx);
+    if (!b) return byte_idx; // Termination byte of string
+    if (utf8_is_start_byte_of_char(b)) {
+      char_idx++;
+      if (char_idx == charnum + 1) return byte_idx;
+    }
+    byte_idx++;
+  }
+}
+
+uint8_t utf8_byte_pos_by_char_num(const char *pstart, const uint8_t charnum) {
+  return utf8_byte_pos_by_char_num_cb(pstart, read_byte_ram, charnum);
+}
+
+uint8_t utf8_byte_pos_by_char_num_P(PGM_P pstart, const uint8_t charnum) {
+  return utf8_byte_pos_by_char_num_cb(pstart, read_byte_rom, charnum);
+}

Marlin/src/lcd/fontutils.h

@@ -41,3 +41,7 @@ uint8_t* get_utf8_value_cb(uint8_t *pstart, read_byte_cb_t cb_read_byte, wchar_t
 /* Returns length of string in CHARACTERS, NOT BYTES */
 uint8_t utf8_strlen(const char *pstart);
 uint8_t utf8_strlen_P(PGM_P pstart);
+
+/* Returns start byte position of desired char number */
+uint8_t utf8_byte_pos_by_char_num(const char *pstart, const uint8_t charnum);
+uint8_t utf8_byte_pos_by_char_num_P(PGM_P pstart, const uint8_t charnum);

Marlin/src/lcd/marlinui.cpp

@@ -208,11 +208,20 @@ millis_t MarlinUI::next_button_update_ms; // = 0
             filename_scroll_pos = 0;                                       // Reset scroll to the start
             lcd_status_update_delay = 8;                                   // Don't scroll right away
           }
+          #if ENABLED(UTF_FILENAME_SUPPORT)
+            // Advance byte position corresponding to filename_scroll_pos char position
+            outstr += utf8_byte_pos_by_char_num(outstr, filename_scroll_pos);
+          #else
             outstr += filename_scroll_pos;
+          #endif
         }
+      #else
+        #if ENABLED(UTF_FILENAME_SUPPORT)
+          theCard.longFilename[utf8_byte_pos_by_char_num(theCard.longFilename, maxlen)] = '\0'; // cutoff at screen edge
         #else
           theCard.longFilename[maxlen] = '\0'; // cutoff at screen edge
         #endif
+      #endif
     }
     return outstr;
   }
@@ -1007,7 +1016,10 @@ void MarlinUI::update() {
       // cause a refresh to occur until all the text has scrolled into view.
       if (currentScreen == menu_media && !lcd_status_update_delay--) {
         lcd_status_update_delay = 4;
-        if (++filename_scroll_pos > filename_scroll_max) {
+        filename_scroll_pos++;
+        if (filename_scroll_pos == filename_scroll_max){
+          lcd_status_update_delay = 12;
+        } else if (filename_scroll_pos > filename_scroll_max) {
           filename_scroll_pos = 0;
           lcd_status_update_delay = 12;
         }