Problem solved! It turns out that some of the resources being loaded by the IFRAME were being loaded via HTTP. The no-referrer-when-downgrade error was the hint that something fishy was going on.
Now we can embed our IDE inside Discourse posts - which will be a HUGE boon to teachers who have questions about the programming activities in the curriculum. Thanks so much to everyone who chimed in to help! I’ve attached a screenshot of a working demo.
