UserAgent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:64.0) Gecko/20100101 Firefox/64.0

Steps to reproduce the problem:
--- from an extension content script ---
1. Create an iframe pointing at about:blank. Load a simple script in it, which I'll refer to as 'sandbox' from here on
--- from the sandbox ---
2. register a 'message' event listener on its parent window
--- from the content script ---
3. Create a MessageChannel, store it. Port1 of this channel will be referred to as 'contentToPage' and port2 will be referred to as 'pageToContent'
4. Add a message event listener to port1 aka contentToPage that console.logs the event data.
5. Transfer port2 aka pageToContent to the sandbox via window.postMessage.
6. Perform a window.setTimeout to schedule an operation I'll refer to as 'detach', defined further below. Give it a moderate delay, like 1000ms
7. Perform a window.setTimeout to schedule an operation I'll refer to as 'chat', defined further below. Give it a long delay, like 5000ms
--- from the sandbox ---
8. Receive pageToContent from the content script. Store it.
9. Unregister your message event listener from the parent window.
10. Add an event listener to pageToContent that console.logs the event data.
11. postMessage "hello world" to the content script through pageToContent.
--- from the content script ---
12. the event listener should log "hello world" because it received it from the sandbox.
13. 'detach' should be triggered by setTimeout. It removes the iframe from the page. This should have no observable effect other than the removal of the iframe.
14. 'chat' should be triggered by setTimeout. it postMessages "hello sandbox" to the sandbox, via contentToPage.
--- from the sandbox ---
15. the sandbox's event listener should log "hello sandbox" because it received it from the content script.

What is the expected behavior?
Event listeners on both sides of the content/page boundary should remain functioning as long as the channel's ports are open

What went wrong?
In every version after 70, the event listener from the sandbox is silently detached from the port and never receives events again.

Both ports are still open and you can postMessage through them. Nothing has been garbage collected.

Did this work before? Yes Chrome v70

Does this work in other browsers? Yes

Chrome version: Version 72.0.3619.0 (Official Build) canary (64-bit)  Channel: canary
OS Version: 10.0
Flash Version: 

It's intended and well documented that unloading an iframe will basically detach its Window object, disabling most of its APIs. That's fine. The parent window object remains functioning, as do its APIs, and that's sufficient. All of the functions and objects created in the iframe remain valid as long as they are retained by the parent window (in a local variable, for example). This works in both 70 and later versions.

The issue specifically appears to be event listener lifetime. The ports are not erroneously being closed or disentangled, the event listeners on them just go away. BroadcastChannel has the same problem. Event listeners created by JS in the parent window remain registered and functioning.

I'm going to try to create a reduced test case here, but naturally, it's very complex to reproduce. This breaks my extension (w/ ~70k active weekly users) presently though I've come up with an elaborate workaround for the GC being broken.

The workaround I've identified that clearly demonstrates the problem is essentially to change this:

port.addEventListener("message", myEventHandler);

to this:

parentWindow.port = port;
parentWindow.myEventHandler = myEventHandler;
parentWindow.eval("port.addEventListener('message', function (evt) { myEventHandler(evt); }");