vterm_screen_get_cell
if (vterm_screen_get_cell(fTermScreen, pos, &cell) != 0)
vterm_screen_get_cell(fTermScreen, firstPos, &cell);
int vterm_screen_get_cell(const VTermScreen *screen, VTermPos pos, VTermScreenCell *cell);
vterm_screen_get_cell(screen, pos, screen->sb_buffer + pos.col);