shadowRoot is null in Jest tests

I’m using @ionic/react@5.3.1 with @testing-library/react@10.4.9 to render an IonButton element.

In the browser, I can do document.querySelector('ion-button').shadowRoot to access the shadow DOM. However, in my Jest tests, the same thing returns null.

I have Jest configured to use jest-environment-jsdom-sixteen, which uses jsdom 16.2.2, so it should have shadow DOM support.