lit-element-click-outside is a @clickOutside decorator that allows you to call component method as the user clicks outside of the host component. Obviously it's very easy. Why then import it as external library? First, because it does not make sense write this boilerplate code in each project. Second, because decorator solution is elegant and convenient.

In your Lit Element project, add lit-element-click-outside to your package.json:

npm i lit-element-click-outside

It's very simple: you just need to anotate your method with @clickOutside and it will be called when user clicks outside of component area.

import { LitElement, html, property, customElement } from 'lit-element';
import { clickOutside } from 'lit-element-click-outside';

@customElement('simple-greeting')
export class SimpleGreeting extends LitElement {
  @property() name = 'World';

  @clickOutside()
  someMethod() {
    console.log(
      "someMethod was called because user just clicked outside of SimpleGreeting"
    );
  }

  render() {
    return html`<p>Hello, ${this.name}!</p>`;
  }
}

You may pass some optional parameters to decorator (or util function):

@clickOutside({
    exclude: 'button .exclude-click-outside-class'
})