Merge pull request #78968 from bruvzg/scroll_fix2

[RTL] Fix multithreaded scrollbar visibility update 2.

scene/gui/rich_text_label.cpp

@@ -2928,6 +2928,7 @@ void RichTextLabel::_process_line_caches() {
 		}
 	}
 
+	total_height = (fi == 0) ? 0 : _calculate_line_vertical_offset(main->lines[fi - 1]);
 	for (int i = fi; i < (int)main->lines.size(); i++) {
 		total_height = _shape_line(main, i, theme_cache.normal_font, theme_cache.normal_font_size, text_rect.get_size().width - scroll_w, total_height, &total_chars);
 		total_height = _update_scroll_exceeds(total_height, ctrl_height, text_rect.get_size().width - scroll_w, i, old_scroll, text_rect.size.height);