Proposal: event listeners registered by content scripts should have higher priority than page's own

[bershanskiy]

Background

When JS registers a listener for a particular event on a particular object, that event listener is appended to the end of target's event listener list.[1] When event is being dispatched, the browser iterates over the list in order and fires events in order. In practice, this gives a rise to a race condition between page's own event listener registrations and extension's own.

Problem

Some pages intentionally cancel some events. Extensions which want to observe a particular event on the page despite that event being cancelled have to set run_at value to document_start and immediately register an event listener on document with capture set to true. Running a large script at document_start is typically bad for performance while registering a multiple content scripts (a dedicated script only to intercept event, and then also the regular script) increases complexity.

Proposal

Event handlers registered by extension on a particular target run before all other event handlers registered by the page.

[1] https://dom.spec.whatwg.org/#ref-for-dom-eventtarget-addeventlistener%E2%91%A2

[tophf]

It must be optional to avoid subtle changes in behavior for extensions that want to listen at the end of the dispatch chain. For example Tampermonkey/Violentmonkey listens to DOMContentLoaded to wait for the page to finish its DOMContentLoaded listeners and change DOM so that the userscripts will run afterwards and see a more finalized version of DOM.

It can be a parameter in addEventListener (Chrome team dislikes this usually) or a dedicated chrome.dom.setListenerPriority calling which would synchronously change the mode for all subsequent addEventListener calls.