Proposal: expose more mv3 APIs to mv2 extensions

[carlosjeurissen]

Currently some new mv3 changes like the object notation for content_security_policy and background.service_worker are also available in mv2.

To ease transition, it would be very helpful if mv3 APIs can be exposed to mv2 extensions as well. There might be specific technical reasons why this hasn't been done before tho, if so, I would love to hear.

Some APIs which would benefit here are the new scripting API, like scripting.registerContentScripts. Also the action.getUserSettings of the action API would be very helpful for browserAction to get the pinned state of extensions.

Feature detection can then be used to check if it's available. Something like:

if (browser.scripting) {
  
} else {
  // use some fallback like browser.tabs.executeScript
}

This raises the question why for some of these APIs a new manifest version is required in general.

[xeenon]

Safari is shipping support for browser.scripting and service_worker support in v2 and v3 extensions in Safari 15.4. We did not version lock anything other than the browser.action rename.