previousNode() considered exactly one candidate — the previous sibling's
deepest last child — and returned null as soon as the filter rejected it,
so a backward walk died mid-tree. This is the backward mirror of #432,
which was fixed forward-only in #447.
<div id=root><a><b></b></a><c></c></div>, SHOW_ELEMENT, B -> FILTER_SKIP
forward: ["A","C"] backward: [] (Chrome: ["A"])
Replaced with the DOM 6.1 algorithm verbatim: scan previous siblings,
descend to the deepest last descendant of each, then step up to the
parent, continuing until a node is accepted or root is reached. The
descent stops on FILTER_REJECT, which is what makes rejection prune
subtrees in the backward direction too.
Unlike nextNode this stays in JS rather than adding a DOM traversal op:
the descent has to consult the filter at every step to honour REJECT, so
there is no run of JS/native crossings for a native helper to collapse.
Depends on the three-valued `_filter` from #461.
Andrey Mnatsakanov
a8c0a19f67