modifying-taxonomic-filter

name: modifying-taxonomic-filter description: Guides safe modification of the TaxonomicFilter — PostHog's multi-tab picker for events, actions, properties, cohorts, and more. Front-loads the empirical product reality (what users actually pick and search for) plus the three live variants (legacy-control, legacy-pill behind TAXONOMIC_FILTER_CATEGORY_DROPDOWN, and the opt-in rebuild menu behind TAXONOMIC_FILTER_MENU_REBUILD) so changes are judged against real behavior and mirrored across surfaces, not made against one arm in isolation. Use when adding features, fixing bugs, or refactoring TaxonomicFilter, the rebuild menu, or the headless filter panel.

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

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

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.

• 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>. Call sites that build their own popover (e.g. ActionFilterRow) never see the rebuild — so "does this reach the rebuild?" depends on the call site, not a single global switch.

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).

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

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/