For everyone dealing with this issue, keep in mind even in mv3 one can use a non-persistent debug-able background page. To do so, use this type of declaration: { "background": { "service_worker": "background.js", "scripts": ["background.js"] } } This will use a serviceWorker in Chrome, while it will use a background script in Safari and Firefox.

This issue has since been resolved.

Can not reproduce this in Safari 18.4. Is this still an issue? Did other extension APIs other than the webNavigation API return tabId of 0 instead of -1?

It happens both on new and existing projects. Extensions load fine in the iOS simulator 18.3 even on Xcode 16.3. So the issue seems to be specifically related to iOS 18.4.

Same issue in Xcode 16.3 release 16E140

A sample extension can be found here: https://github.com/carlosjeurissen/webext-tech-demo-extensions/tree/main/issues/safari-9723963-merged-sync-and-local-storage And the feedback assistant report can be found here: https://feedbackassistant.apple.com/feedback/9723963