With respect to the Extension API calling `this.sendMessage` is not resulting in the iframe receiving the message. The code is being called from a class that extends `Panel`. The receiving iframe _does_ receive messages from the parent frame but not the ones I'm sending.
Here's some of the `extension.ts` code: https://pastebin.com/LnTGjp9C
The receiving frame's code is simple:
window.addEventListener('message' function(event) {
console.log(arguments);
});
Here's a screenshot of the events that _are_ captured (but not the one I'm sending from the parent):
