The HTML Processor maintains a stack of open elements, where every element,
every `#text` node, every HTML comment, and other node is pushed and popped while
traversing the document. The "depth" of each of these nodes represents how deep
that stack is where the node appears. Unfortunately this information isn't
exposed to calling code, which has led different projects to attempt to
calculate this value externally. This isn't always trivial, but the HTML
Processor could make it so by exposing the internal knowledge in a new method.
In this patch the `get_current_depth()` method returns just that. Since the
processor always exists within a context, the depth includes nesting from the
always-present html element and also the body, since currently the HTML
Processor only supports parsing in the IN BODY context.
This means that the depth reported for the `DIV` in `<div>` is 3, not 1, because
its breadcrumbs path is `HTML > BODY > DIV`.
Developed in https://github.com/WordPress/wordpress-develop/pull/6589
Discussed in https://core.trac.wordpress.org/ticket/61255Fixes#61255.
Props dmsnell, jonsurrell.
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