[Bug 27191] New: "callback interface NodeFilter"

https://www.w3.org/Bugs/Public/show_bug.cgi?id=27191

            Bug ID: 27191
           Summary: "callback interface NodeFilter"
           Product: WebAppsWG
           Version: unspecified
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DOM
          Assignee: [email protected]
          Reporter: [email protected]
        QA Contact: [email protected]
                CC: [email protected], [email protected]

https://dom.spec.whatwg.org/#interface-nodefilter

[[
callback interface NodeFilter
]]

I think this is wrong, or at least confusing. Looking at
https://heycam.github.io/webidl/#dfn-callback-interface,

> Callback interfaces are ones that can be implemented by user objects and not by platform objects, as described in section 3.9 below. 

Also

> Static attributes and static operations MUST NOT be defined on a callback interface. 

Which doesn't specifically call out `const`s, but seems the same in spirit.

Additionally, while `window.NodeFilter` exists in all browsers,
`window.EventListener` does not, and earlier the spec declares `callback
interface EventListener` very similarly. I think (but am not sure) per WebIDL
callback interfaces don't exist globally.

Suggested correction, if I am understanding this correctly:

- Add a separate `interface NodeFilter` that shows up on the global, has
constants, and has an `acceptNode` method.
- Add `callback interface NodeFilterCallback` that contains only `acceptNode`
and is used by createNodeIterator and createTreeWalker.
- Specify that passing a `NodeFilterCallback` (which could just be a plain
function, per the definition of callback interfaces) to e.g. createNodeIterator
causes the creation of a new `NodeFilter` object that is reflected by the
`filter` property of the created node iterator.

Does this make sense? Am I on to something or just missing the idea entirely?

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