code: def scroll_page_down(event):
      
      w = _current_window_for_event(event)
      b = event.cli.current_buffer
  
      if w and w.render_info:
          # Scroll down one page.
          line_index = max(w.render_info.last_visible_line(), w.vertical_scroll + 1)
          w.vertical_scroll = line_index
  
          b.cursor_position = b.document.translate_row_col_to_index(line_index, 0)
          b.cursor_position += b.document.get_start_of_line_position(after_whitespace=True)
docs: Scroll page down. (Prefer the cursor at the top of the page, after scrolling.)