Shadow DOM problems for extensions

[lapcat]

Websites are increasingly using the shadow DOM for complex DOM hierarchies. Reddit is an example of a site that makes heavy use of the shadow DOM. I've also seen sites that are putting <video> elements inside the shadow DOM.

It's possible to override Element.attachShadow(), and this works ok, but the declarative shadow DOM is a nightmare for extensions. I understand that Chrome has chrome.dom.openOrClosedShadowRoot() and Firefox has Element.openOrClosedShadowRoot, but Safari has nothing, and in any case it's not performant to check every element in the document for potential shadow roots.

Here's a list of things my extension needs to do with shadow roots:

1. Add event listeners
2. Add styles
3. querySelectorAll()
4. MutationObserver.observe()

It would be great if there were some convenient and performant way for extensions to find shadow roots, for example if querySelectorAll() and MutationObserver.observe() outside the shadow DOM had an option to include shadow roots.

The shadow DOM is intended for encapsulating code within a site, but I don't think it's intended to make a site off-limits to extensions.

[polywock]

I understand that Chrome has chrome.dom.openOrClosedShadowRoot() and Firefox has Element.openOrClosedShadowRoot, but Safari has nothing,

On a related note, this browser inconsistency should be worked on. Maybe browser.dom should become a standard namespace.

[Rob--W]

While the request seems reasonable, I'm afraid that implementation complexion limits the feasibility of implementing your requested functionality, at least for querySelector and querySelectorAll.

The ability to look through the shadow DOM boundaries has come up many times before on the web platform, including use cases such as automation / testing. Search terms to use are "pierce" / "piercing" / "deep".

Examples:

"Support >>> combinator in static profile"
WICG/webcomponents#78

"Provide a shadow DOM-piercing querySelector and querySelectorAll for extensions" (https://bugzilla.mozilla.org/show_bug.cgi?id=1475869)

[lapcat]

It would be sufficient, I think, if there was some kind of event that could be observed whenever an element gets a shadowRoot, regardless of whether that's via attachShadow or shadowrootmode. Upon observing such an event, the extension could take care of everything else itself.

A piercing querySelector would be nice but not necessary.

[dotproto]

Thinking out loud for a moment, perhaps we should make a point of collecting CSS issues related to WebExtensions so that we can bring them to the CSSWG. Perhaps if we have some specific use cases that we think should be supported in extensions if not the broader web, they would be open to working with us to integrate user agent-only allowances into the relevant specs.

[Rob--W]

Note #624 covers the event listener use case of this request.