Exchange of messages through the new Offscreen Document API.

[Robbi]

Hi developers,
I have an extension I'm trying to convert (with no little effort) to MV3.
This extension modifies the home page of the browser with the directive:
"chrome_url_overrides": { "newtab": "newpage.html" },

I noticed a weird thing using the new Offscreen Document API.

From the SW, on several occasions I need to parse the DOM of an html page that I receive as a response from a fetch command.
When I exchange a message between sender (service worker) and recipient (the offscreen document), that message is delivered to the offscreen document and I get the desired response,
but I noticed that that message
also reaches the runtime.onMessage event handler of the extension page (the new tab) and sometime also reaches the  runtime.onMessage event handler of the SW.

The question I ask you at this point is simple. Is this normal behavior?

Using  runtime.sendMessage shouldn't I get the response only in its callback function or as a return value if I use the promise syntax?

A Minimal, Reproducible Example can be found here:
gitHub Rep

[wOxxOm]

Yes, it's the correct behavior and it always worked like this: chrome.runtime.sendMessage is broadcasting to the entire origin of chrome-extension://id so it reaches every page, every popup, every web_accessible_resources iframe in addition to the background script.

Until a better API is created you can use the standard SW messaging via navigator.serviceWorker in the client (newtab script, offscreen document) + self.onmessage listener in SW. There's also the reversed direction: the SW can find the client via clients.matchAll() then use postMessage on the found client, while the client would use navigator.serviceWorker.onmessage to listen to the messages.

[Robbi]

Thanks you wOxxOm,

to send a message to an extension page I've always used tabs.sendMessage,

however I was convinced that runtime.sendMessage could only communicate with background scripts, SW and action popups.

You gave me ideas to think about...

[Robbi]

I was wondering if it hadn't been better think the OSD as a hidden tab with a given tabId (as Firefox seems to me to do) so that messages could only reach the OSD.
Of course, that's just my thought.

[Jordan Cohen]

I am pretty sure that this is what is says in the docs too... I didn't think it was even possible for 2 content scripts to talk to each other without a background script brokering the message...

[wOxxOm]

This topic is not about content scripts though.