ensureTooltipVisible method
Shows the tooltip if it is not already visible.
After made visible by this method, The tooltip does not automatically
dismiss after waitDuration, until the user dismisses/re-triggers it, or
Tooltip.dismissAllToolTips is called.
Returns false when the tooltip shouldn't be shown or when the tooltip
was already visible.
Implementation
bool ensureTooltipVisible() {
  if (!_visible) {
    return false;
  }
  _timer?.cancel();
  _timer = null;
  if (_controller.isForwardOrCompleted) {
    return false;
  }
  _scheduleShowTooltip(withDelay: Duration.zero);
  return true;
}