Add 'hierarchy' attribute to taxonomy filter

[goldenapples]

Adds options for "flat" (default), "collapsed", or "nested" display of hierarchical taxonomies.

[github-actions]

Playwright — PHP 8.2 / WP latest

 34 passed

Details

 34 tests across 8 suites
 35.8 seconds
 23544ca

[kadamwhite]

Looks good to my human eyes, and Claude likes it too. Agent code review in details tag below

Click to expand agentic review notes (none blocking)

Correctness (verified)

• Selecting a parent matches its children. This comes from WordPress's include_children default. The
  plugin sets 'operator' => 'IN' for multiple values (inc/namespace.php:123) and the default operator
  for a single value. AND would have broken it, because WP expands the children into the term list and
  then requires all of them. Nothing in the PR depends on it, but a comment near the tax_query would
  stop someone switching to AND later.
• Parents with no posts of their own still show. get_terms() keeps empty parents of non-empty children
  when hide_empty is on and hierarchical is left at its default of true. The Networking fixture
  exercises this correctly.
• Collapsed-mode context nesting is correct. Each list below the top level reads expanded from its
  enclosing
• , and a grandchild's own context shadows its parent's. Server-side directive processing
  renders hidden and aria-expanded="false" on first paint, which the tests rely on.
• Escaping is fine. In render.php:570, $item_attributes is built only from literal strings and
  esc_attr( wp_json_encode() ), so the phpcs ignore is justified.

Design points to discuss

1. maxVisibleTerms only counts top-level terms in nested and collapsed modes (render.php:435). With one
   parent holding 40 children, the cap never kicks in. This is deliberate and commented, but editors
   won't expect it. Either note it in the control's help text, or accept it on the grounds that
   collapsed mode covers this case.
2. Checking a parent and one of its children gives a redundant selection. The query string becomes
   networking,sd-wan, which returns the same results as the parent alone. Checking a parent also
   doesn't show the children as covered. It isn't a bug, but the checkbox mental model is a bit off.
   Radio doesn't have the problem.
3. Overflow now differs by radio mode. Nested and collapsed radio get the "See all" overflow through
   $render_branch, but flat radio (render.php:267-273) still loops over $terms with no overflow
   handling, as before. Either close the gap or treat it as out of scope.
4. The [hidden] CSS rule fixes an existing bug (filters.css:124).
   .wp-block-query-filter__checkbox-group label { display: flex } already beat the browser's [hidden]
   styling on main. That means the existing checkbox overflow toggle never actually hid anything, and
   no e2e test covers overflow. The fix is correct, but it belongs in the PR description, and possibly
   in a regression test.

Smaller issues

• The select control shows multiple selections differently now (render.php:618). It used to compare
  against the raw $current_value, which selected nothing when the value was a comma list. It now marks
  every matching option selected, and the browser shows the last one. Flat radio changed the same way.
  This only matters if two filters share a query var, so it's negligible, but it is a behavior change.
• Tree helpers are rebuilt on every render (edit.js:96-115). buildTree and flattenTree are pure and
  could live at module scope.
• The > label locator may not be valid (taxonomy-hierarchy.spec.js:913). I'm not sure Playwright
  accepts a leading combinator. :scope > label definitely works. Please confirm this test actually ran
  green.
• The "starts open" test is incomplete (taxonomy-hierarchy.spec.js:859). It checks aria-expanded and
  the checked input but not that the child list is visible. Add toBeVisible(), since visibility is what
  the CSS fix makes work.
• The inherited-search assertion is weaker than before (inherited-search.spec.js:724). Relaxing it is
  reasonable: the new fixture pages now appear in any-post-type search results ahead of the older
  posts. Still, the new assertion only proves the search isn't "Alpha only".
• Some combinations have no tests: radio with nested, overflow combined with a hierarchy, and
  horizontal layout with a hierarchy.

Not flagged

The build_term_tree and flatten_term_tree recursion is O(n²). That doesn't matter here, because
get_terms is capped at 100 terms (namespace.php:210).

[kadamwhite]

Nice bugfix, in the midst of this