createNodeIterator returned a TreeWalker with a flag on it. The two are
different interfaces, and the alias got two things wrong.
An iterator's pointer starts *before* its root, so the first nextNode()
returns the root itself; a TreeWalker starts *at* its root and never
returns it. So createNodeIterator(el, SHOW_ELEMENT) silently skipped
exactly the element it was rooted at:
<div id=root><a></a></div> -> ["A"] (Chrome: ["DIV","A"])
It also exposed the wrong surface: no referenceNode,
pointerBeforeReferenceNode or detach (older library code still calls
detach and hit "not a function"), while leaking currentNode, firstChild,
parentNode and the rest of TreeWalker's movers.
Implemented DOM 6.2 "traverse" directly. Keeping the pointer as
(referenceNode, pointerBeforeReferenceNode) rather than a single cursor
is what makes a direction change re-yield the current node instead of
stepping over it, and a failed traversal leaves both untouched so the
iterator stays resumable either way.
The backward step is a new `DomTree::prev_in_subtree`, mirroring
`next_in_subtree`. It can live entirely in the DOM layer because a
NodeIterator prunes nothing -- unlike the TreeWalker's backward walk
(#462), where FILTER_REJECT forces the filter to be consulted at every
step of the descent.
FILTER_REJECT still behaves as FILTER_SKIP here, which the existing test
keeps covering; its expectation gains the root node the alias used to
drop. The TreeWalker's now-dead `_rejectIsSkip` branch is removed.
Andrey Mnatsakanov
ab3ca26046