Beware that connectedCallback runs _every time_ a custom element is added to the dom. So you should make sure to only add event listeners once by tracking internally if the element was already initialized.
Thanks, that's good to know. In some contexts I don't think a custom element is ever added to the DOM more than once, eg in htmx I am responding with HTML markup which is then injected into the page and then just kept there, possibly 'forever' or at least until it is replaced by some new markup from a new response.