HTMLElement with contentAvailableCallback (483 bytes / 317 gzip / 252 brotli, CC0-1.0)

HTMLElement-contentAvailableCallback.js

{
	let NativeHTMLElement = globalThis.HTMLElement, isContentAvailable = (element) => {
		if (element.ownerDocument.readyState !== 'loading') return true

		while (element) {
			if (element.nextSibling) return true

			element = element.parentElement
		}
	}
	(
		(
			globalThis.HTMLElement = Object.setPrototypeOf(function HTMLElement() {
				let host = Reflect.construct(NativeHTMLElement, [], new.target), { contentAvailableCallback } = host
				if (typeof contentAvailableCallback === 'function') {
					if (isContentAvailable(host)) contentAvailableCallback.call(host)
					else new MutationObserver((record, observer) => {
						if (isContentAvailable(host)) {
							observer.disconnect()

							contentAvailableCallback.call(host)
						}
					}).observe(host, { childList: true })
				}
				return host
			}, Element)
		).prototype = NativeHTMLElement.prototype
	).constructor = HTMLElement
}

HTMLElement-contentAvailableCallback.min.js

{let e=globalThis.HTMLElement,t=e=>{if("loading"!==e.ownerDocument.readyState)return!0;for(;e;){if(e.nextSibling)return!0;e=e.parentElement}};((globalThis.HTMLElement=Object.setPrototypeOf((function HTMLElement(){let n=Reflect.construct(e,[],new.target),{contentAvailableCallback:l}=n;return"function"==typeof l&&(t(n)?l.call(n):new MutationObserver(((e,o)=>{t(n)&&(o.disconnect(),l.call(n))})).observe(n,{childList:!0})),n}),Element)).prototype=e.prototype).constructor=HTMLElement}