[Static content optimization] <iframe> onload events may not fire

[nolanlawson]

Description

The onload event does not fire for an <iframe> without a src because the event listener is added after the <iframe> has already been inserted into the DOM. This only occurs when the <iframe> is optimized by the static content optimization.

Steps to Reproduce

Minimal repro

<template>
  <iframe onload={loadHandler}></iframe>
  <span if:true={loaded}>show me</span>
</template>

import { LightningElement } from 'lwc';

export default class extends LightningElement {
    loaded = false;

    loadHandler() {
        console.log('load handler called');
        this.loaded = true;
    }
}

Expected Results

onload handler fires

Actual Results

onload handler does not fire

Possible Solution
Adding data-foo={foo} to the iframe disables the static content optimization, and the bug goes away.