[proposal] Node.containingShadowRoot

[emilio]

I think this would be useful for authors that want to write Shadow-DOM aware code.

All browser engines have a way to get a hand to the subtree root and check if it's a shadow root in one way or another.

Gecko: https://searchfox.org/mozilla-central/rev/2e355fa82aaa87e8424a9927c8136be184eeb6c7/dom/base/nsINode.cpp#699

WebKit: https://trac.webkit.org/browser/webkit/trunk/Source/WebCore/dom/Node.cpp?rev=+254087#L1127

Chromium: https://source.chromium.org/chromium/chromium/src/+/master:third_party/blink/renderer/core/dom/node.cc;l=1815;drc=95e78e2d51fce9616e316073be6b2b92189e21ee

How do folks feel about this? Maybe it should just be in Element and Text or something...

cc @whatwg/components

[caridy]

@emilio can you provide more details and use-cases for this? Just curious how is this different from accessing the root node? what should be the behavior when the element is disconnected, etc?

[emilio]

It would not different to accessing the root node, but it can be O(1) rather that going through all the parents.

I just got a case where I wanted to check the activeElement of the containing tree, so (element.containingShadowRoot || document).activeElement.

[emilio]

I guess getRootNode() would also work, kinda. You wouldn't have the "is a shadow root" check built-in but that seems acceptable.

[annevk]

If we wanted to we could add some kind of filtering to that method via a dictionary member, but not sure if it’s worth the trouble. Depends a bit on how often it comes up I suppose.