Modifying the TaxonomicFilter

The TaxonomicFilter is the picker users hit to choose any "thing PostHog
knows about" — events, properties, actions, cohorts, groups. It's the
on-ramp into almost every analytics and replay configuration. Code lives
in frontend/src/lib/components/TaxonomicFilter/.

Two unbreakable rules:

1. Changes that demote items users actually pick are regressions, even
   with all tests passing. Read "Product reality" before deciding any
   change is safe. Ordering, promotion, or position-0 changes need
   explicit human sign-off — don't let an agent decide alone.
2. There are three live variants behind two feature flags, and the
   rebuild is a parallel reimplementation of the legacy data + group
   layer — not a skin over it. A behaviour change usually has to land in
   both the legacy code and the rebuild, or the two arms of the
   experiment diverge. Read "Three variants" and "Mirroring changes"
   before assuming one edit is enough.

Product reality (last refreshed 2026-05-02, 90-day window)

Ratios from production telemetry. Re-run via
references/refreshing-product-reality.md
when older than ~3 months.

How users pick

• Top three rows carry ~80% of selections (position 0: ~56%,
  position 1: ~15%, position 2: ~8%). Demoting a popular item out of
  the top three is a real-user regression.
• ~34% selection rate. Two of every three opens close without a
  pick. p50 dwell ~7s, p90 ~53s — most opens are quick glances.
• Selection paths: ~65% via search, ~19% browsed-no-search,
  ~16% from recents, <1% from pinned items.

What users select

Source group type	Share
events	~40%
event_properties	~30%
person_properties	~14%
cohorts	~2%
email_addresses	~2%
actions	~2%
pageview_urls	~1%
everything else	<1%

What users search for (share of top-8 terms)

Term	Share
email	~29%
url	~22%
user	~12%
utm	~10%
page	~9%
path	~8%
current	~6%
country	~5%

email and url are over half the top-8. They're the entire reason
PROMOTED_PROPERTIES_BY_SEARCH_TERM (in infiniteListLogic.ts) maps
them to $email and $current_url at position 0. Touching
promotion or ordering needs explicit human sign-off.

Empty searches

email, url, utm, path against cohorts, event_feature_flags,
session_properties produce most empty-result events — users type the
same canonical terms across every tab. Tab order, suggested-filters
aggregation, and shortcut routing are how they get to the right answer.

Input mode

~93% typed, ~7% pasted. Both feed inputMode on taxonomic_filter_search_query.

Telemetry is a contract

Treat property shapes as a public API. Every taxonomic filter * event now
carries a surface property (legacy-control / legacy-pill /
rebuild-menu) so the experiment arms are distinguishable by an explicit
property, not a feature-flag join. The legacy stamp comes from
legacyTaxonomicSurface() in taxonomicFilterSurface.ts; the rebuild stamps
rebuild-menu from menu/TaxonomicFilterMenu.tsx.

Shared events both surfaces emit (keep these comparable across arms):

• taxonomic filter closed — surface, dwellMs, hadSelection (legacy
  also sends groupType; the rebuild omits it — there's no single active
  tab at close)
• taxonomic filter item selected — surface, groupType,
  sourceGroupType, wasFromRecents, wasFromPinnedList, wasQuickFilter,
  hadSearchInput, position, query, wasStale

Legacy-only: taxonomic_filter_search_query
(searchQuery, groupType, inputMode, pastedFraction),
taxonomic filter empty result (groupType, searchQuery),
taxonomic filter include stale toggled,
taxonomic filter category dropdown opened (pill only).

Rebuild-only menu events: taxonomic filter menu opened / drilled /
closed / option clicked / item selected.

When you add a property to a shared event, add it to both emitters
or the arms stop being comparable. Adding properties: fine. Removing dead
ones: fine. Renaming or repurposing silently is the worst case —
dashboards keep working and start lying.

Three variants

Two feature flags, three surfaces. A bug report that doesn't reproduce
locally is almost always a variant mismatch — confirm which surface the
reporter is on first.

Surface	Flag	Value	What renders
legacy-control	TAXONOMIC_FILTER_CATEGORY_DROPDOWN	'control'	original tab-pill UI
legacy-pill	TAXONOMIC_FILTER_CATEGORY_DROPDOWN	'pill'	suffix category dropdown (CategoryDropdown.tsx)
rebuild-menu	TAXONOMIC_FILTER_MENU_REBUILD	on	ground-up rewrite in menu/ over headless/

• legacy-control vs legacy-pill is the same A/B we've always had —
  one codebase (taxonomicFilterLogic.tsx + InfiniteList), two render
  paths. Owner @pauldambra, multivariate control,pill. The direction
  of travel is to move everyone from control onto pill.
• rebuild-menu is a separate, opt-in experiment (@adamleith) being
  tested internally. It is a fresh implementation: the menu/ dropdown
  and combobox UI on top of headless/ (a hooks-based filter panel). It
  does not route through taxonomicFilterLogic/infiniteListLogic;
  it has its own group definitions, fetch/pagination, and ordering. See
  headless/UX_SPEC.md for its design source of truth.

The rebuild is opt-in in exactly two consumer wrappers:
TaxonomicPopover.tsx and
PropertyFilters/components/TaxonomicPropertyFilter.tsx. Both check
TAXONOMIC_FILTER_MENU_REBUILD and render <TaxonomicFilterMenu> or the
legacy <TaxonomicFilter>. A call site reaching one of those wrappers can
still land on the legacy path: TaxonomicPopover renders the rebuild only
when newMenuSupportsCallSite holds (!allowClear && closeOnChange && ref == null), because the rebuilt menu cannot honour those three capabilities.
So "does this reach the rebuild?" depends on the wrapper and the props
that call site passes, not a single global switch — ActionFilterRow goes
through TaxonomicPopover and passes none of the three, so it does reach
the rebuild. Only a call site that hand-rolls its own popover around
<TaxonomicFilter> bypasses the wrappers entirely.

Touching tab/group rendering means testing all three surfaces.

Mirroring changes across variants

The rebuild reimplements the legacy data layer rather than reusing it, so
the same concern lives in two files. There is no lint rule or test
enforcing parity — the only guard is "Mirrors the legacy…" comments. When
you change one, change the other (or flag to the human that you can't).

Concern	Legacy	Rebuild
Group definitions (endpoint, excluded props, group meta)	taxonomicFilterLogic.tsx taxonomicGroups selector	utils/buildTaxonomicGroups.tsx
Group ordering + SuggestedFilters injection	taxonomicFilterLogic.tsx taxonomicGroupTypes selector	hooks/useTaxonomicFilter.ts resolveTaxonomicGroupTypes
Per-tab fetch / pagination / min-query-length	infiniteListLogic.ts	hooks/useGroupList.ts + useTaxonomicResource.ts + fetchTaxonomicListPage.ts
Data-warehouse config flow	inline in InfiniteList.tsx	menu/DwhFlow.tsx
taxonomic filter item selected / closed telemetry	taxonomicFilterLogic.tsx	menu/TaxonomicFilterMenu.tsx
New TaxonomicFilterGroupType enum value	types.ts (shared) — then add group config in both tables above
Logic-backed group data (Actions, Dashboards, …)	already in kea	also register in hooks/useTaxonomicLocalOverrides.ts

Genuinely shared — change once: types.ts (the enum),
utils/promoteProperties.ts (PROMOTED_PROPERTIES_BY_SEARCH_TERM),
recentTaxonomicFiltersLogic.ts and
taxonomicFilterPinnedPropertiesLogic.ts (the rebuild reads recents/pinned
through these via a bridge, it doesn't fork them).

redistributeTopMatches is legacy-only, not shared: it lives in
taxonomicFilterLogic.tsx and the rebuild never calls it.

One intentional divergence is already documented in
useTaxonomicFilter.ts: the rebuild always leads with SuggestedFilters,
whereas legacy gates that on the pill variant. Preserve documented
divergences; don't "fix" them into parity.

Pre-change checklist

• Read references when relevant: architecture,
  common-pitfalls (X/Y matrix),
  call-sites (smoke tests),
  testing-patterns
• Decide whether the change must mirror across legacy and rebuild
  (see "Mirroring changes") — if you can only do one, say so explicitly
• Test all three surfaces if you touched tabs/groups: legacy-control,
  legacy-pill, rebuild-menu
• Confirm shared telemetry payloads still match across both emitters
• Ordering / promotion / position-0 -> human sign-off, not agent judgement
• Flag the ongoing experiments to the human reviewer: the
  control->pill rollout and the internal rebuild-menu opt-in

hogli test frontend/src/lib/components/TaxonomicFilter/