Revised Mastodon embed script, handles dynamically added oembed iframes

mastodon-embed.js

/**
 * Revised by Charles Johnson - https://github.com/GreenFootballs
 *
 * Runs automatically at window.onload(), searches document for iframes with a class of "mastodon-embed"
 * and resizes them to fit the content, using window.postMessage() to get the content height from the
 * embedded page.
 * 
 * Adds an object named "_mastodonTools" to the window element, with an "embed" method.
 * 
 * To resize dynamically added Mastodon iframes, call "_mastodonTools.embed(container, callback)"
 * where "container" is the enclosing element for the dynamic content.
 * 
 * The "container" parameter can be a string with a CSS selector, a reference to a DOM element,
 * or left undefined to search the entire document.
 * 
 * The "callback" parameter is an optional function called after all iframes are resized.
 */
;(function() {
    'use strict'
    window.addEventListener('load', function() {
        window._mastodonTools = {
            iframes: new Map(),
            onRender: null,
            embed: function(container, callback) {
                window._mastodonTools.onRender = null
                if (typeof container === 'string') {
                    container = document.querySelector(container)
                    if (!container) return
                }
                else if (typeof container === 'undefined') {
                    container = document
                }
                var embeds = container.querySelectorAll('iframe.mastodon-embed')
                if (!embeds) return
                Array.from(embeds).forEach(function (iframe, index, arr) {
                    var id = 0, failCount = 0, idBuffer = new Uint32Array(1)
                    while (id === 0 || window._mastodonTools.iframes.has(id)) {
                        id = crypto.getRandomValues(idBuffer)[0] // select unique id for each iframe
                        failCount++
                        if (failCount > 100) { // give up and assign (easily guessable) unique number
                            id = -(window._mastodonTools.iframes.size + 1)
                            break
                        }
                    }
                    window._mastodonTools.iframes.set(id, iframe)
                    iframe.scrolling = 'no'
                    iframe.style.overflow = 'hidden'
                    iframe.onload = function () {
                        iframe.contentWindow.postMessage({ type: 'setHeight', id: id }, '*') // Send message requesting content height
                    }
                    iframe.onload()
                    if (index === arr.length - 1 && typeof callback === 'function') window._mastodonTools.onRender = callback
                })
            }
        }
        window.addEventListener('message', function (e) { // The embedded page replies with the content height
            var data = e.data || {}
            if (typeof data !== 'object' || data.type !== 'setHeight' || !window._mastodonTools.iframes.has(data.id)) return
            var iframe = window._mastodonTools.iframes.get(data.id)
            if ('source' in e && iframe.contentWindow !== e.source) return
            iframe.height = data.height
            if (typeof window._mastodonTools.onRender === 'function') {
                window._mastodonTools.onRender()
                window._mastodonTools.onRender = null
            }
        })
        window._mastodonTools.embed()
    })
})();

Best way to use this is to install it along with the rest of your scripts, so it loads with the page. Mastodon's default oembed code includes their own embed.js script, but it won't work for dynamically added iframes, so you may want to remove it from pasted embed codes.