@jw732 Thanks for your answer. The API I use is for App Extension. So far, I think I can't achieve this goal with App Extension API. Probably it's time for me to try to migrate to Web Extension. Anyways, I tried your code but active in your code doesn't seem to have the error info, but instead, onErrorOccurred seems to work fine like this: browser.webNavigation.onErrorOccurred.addListener(function (details) { ... }); details includes url where the error occurred, so I think I can handle it. Well, I have to migrate my extension to Web Extension first, but still, your answer gave me a good insight for me. Thanks so much.

Thanks bweinstein. My Web extension shows the same content with the app and the user can edit that content in the app. When the user edits it, the app tells the extension so via DistributedNotificationCenter and then the extension applies the changes to background.js and the popup. However, because of this issue, everytime when the user edits, Safari is switched to foreground. I find it annoying when the user wants to stay at the app to keep editing. I don't say the current behavior is a bug, but at least I'd like to have a workaround to avoid this behavior. By the way, you can check the details from "Why this is a problem?" section in FB9804951. Please let me know if I can help you solve this issue 🙂

Now I don't get the concept of notalization. 🙄

I also see the +[NSExtensionContext _allowedItemPayloadClasses] not implemented. Setting the allowed payload classes to {( NSObject )} warning for my QuickLook Preview extension for iOS. At first, I thought this caused the issue where the file preview wasn't showing for my app's file in the Files app. However, after restarting the Files app, the preview displayed correctly, but the warning still appeared in the Console app. So probably this is just an verbose log you can safely ignore.