feat: MPS-24 — chip strip stays on every SERP page

Operator: 'leave the chits on screen for all serp pages.' The
horizontal tag-chip-strip only rendered on the shop home; drilling
into a category (tag-detail SERP) dropped it, so hopping categories
meant going back.

- Extracted the chip strip (duplicated verbatim in home.j2 + shop.j2)
  into a single _facet_nav.j2 chip_strip(...) macro — DRY, one source
  of truth — and added it to shop_tag.j2 under the header.
- shop_tag_detail view already supplied home_chips / active_tag / sort
  / price, so this was a template-only gap. Active category chip
  highlights on the SERP and carries facet_qs (sort/price compose).
- Search SERP renders home.j2 so it gets the macro for free.

Test: +test_chip_strip_stays_on_tag_detail_serp. 1136 passed.
Docs: mps-24.md Phase 2.8i.

docs/tickets/mps-24.md

@@ -363,6 +363,19 @@ Tests (`test_functional.py::TestProductTagsSpa`):
 Deferred (occasional click, not the hot path): AJAX-ifying the
 "Suggest categories" link — still a full navigation by design.
 
+**Phase 2.8i — chip strip on every SERP page** (shipped 2026-05-16):
+operator: "leave the chits on screen for all serp pages." The
+horizontal `tag-chip-strip` rendered only on the shop home; drilling
+into a category (tag-detail SERP `shop_tag.j2`) dropped it, so hopping
+categories meant going back. Extracted the (duplicated) chip strip
+from `home.j2`/`shop.j2` into a single `_facet_nav.j2` `chip_strip(...)`
+macro and added it to `shop_tag.j2` under the header. The
+`shop_tag_detail` view already supplied `home_chips` / `active_tag` /
+sort / price, so this was a template-only gap; the active category
+chip highlights on the SERP and carries `facet_qs`. Search SERP
+already renders `home.j2` so it gets the macro for free. Test:
+`test_chip_strip_stays_on_tag_detail_serp`.
+
 **Phase 2.8h — facets compose, not clobber** (shipped 2026-05-16):
 operator: "switching one breaks it" — picking a category reset the
 active Sort + Price. Cause: the facet category links / "All" link /

make_post_sell/templates/_facet_nav.j2

@@ -118,3 +118,22 @@
   {{ facet_form(base_url, tag_base, sort_options, sort_key, price_min, price_max, facet_tags, active_tag) }}
 </details>
 {% endmacro %}
+
+{# Horizontal category chip strip. Single source of truth — used by the
+   shop home (home.j2 / shop.j2) AND every SERP page (shop_tag.j2) so
+   the chips stay on screen as the shopper hops categories. Carries the
+   active sort/price (facet_qs) so switching category composes. #}
+{% macro chip_strip(shop_url, tag_base, home_chips, active_tag, sort_key, price_min, price_max) %}
+{% if home_chips %}
+<nav class="tag-chip-strip" data-tag-strip aria-label="Browse by category">
+  <a href="{{ shop_url }}{{ facet_qs(sort_key, price_min, price_max) }}"
+     class="tag-chip{% if not active_tag %} tag-chip-active{% endif %}"
+     data-tag-slug="" rel="nofollow">All</a>
+  {% for chip in home_chips %}
+  <a href="{{ tag_base }}/tag/{{ chip.slug }}{{ facet_qs(sort_key, price_min, price_max) }}"
+     class="tag-chip{% if active_tag and active_tag.id == chip.id %} tag-chip-active{% endif %}"
+     data-tag-slug="{{ chip.slug }}" rel="nofollow">{{ chip.name }}</a>
+  {% endfor %}
+</nav>
+{% endif %}
+{% endmacro %}

make_post_sell/templates/home.j2

@@ -88,24 +88,11 @@
      Layout 0 (flat) keeps its existing simple grid with no nav. #}
   {% set show_facet_nav = request.shop.home_layout and request.shop.home_layout >= 1 %}
 
-  {# MPS-24: tag chip strip — shown on layout 1 (chips) and 2 (lanes).
-     Chips and the facet left-nav point at the SAME targets (shop home
-     for "All", {tag_base}/tag/{slug} per category) so the two stay in
-     lock-step. tag_filter.js keeps full navigation for /tag/ hrefs. #}
-  {% if home_chips %}
-  <nav class="tag-chip-strip" data-tag-strip aria-label="Browse by category">
-    <a href="{{ shop_url }}{{ facet.facet_qs(sort_key, price_min, price_max) }}"
-       class="tag-chip{% if not active_tag %} tag-chip-active{% endif %}"
-       data-tag-slug=""
-       rel="nofollow">All</a>
-  {% for chip in home_chips %}
-    <a href="{{ tag_base }}/tag/{{ chip.slug }}{{ facet.facet_qs(sort_key, price_min, price_max) }}"
-       class="tag-chip{% if active_tag and active_tag.id == chip.id %} tag-chip-active{% endif %}"
-       data-tag-slug="{{ chip.slug }}"
-       rel="nofollow">{{ chip.name }}</a>
-  {% endfor %}
-  </nav>
-  {% endif %}
+  {# MPS-24: tag chip strip — shared macro, shown on layout 1 (chips)
+     and 2 (lanes) AND every SERP page (shop_tag.j2). Chips + facet
+     left-nav hit the SAME targets; tag_filter.js keeps full navigation
+     for /tag/ hrefs. #}
+  {{ facet.chip_strip(shop_url, tag_base, home_chips, active_tag, sort_key, price_min, price_max) }}
 
   {% if show_facet_nav %}
   {# Mobile/tablet: collapsed <details> facets above the grid. #}

make_post_sell/templates/shop.j2

@@ -15,27 +15,12 @@
 {% set tag_base = request.shop.absolute_url(request, slug=False) %}
 {% set show_facet_nav = request.shop.home_layout and request.shop.home_layout >= 1 %}
 
-{# MPS-24: tag chip strip — shown on layout 1 (chips) and 2 (lanes). #}
-{# Chip strip and the facet left-nav point at the SAME targets: the
-   shop home for "All" and the canonical tag detail page
-   ({tag_base}/tag/{slug}) per category. tag_filter.js detects the
-   /tag/ href and keeps full navigation, so a chip and its matching
-   sidebar entry always land on the same page with the same active
-   state. #}
-{% if home_chips %}
-<nav class="tag-chip-strip" data-tag-strip aria-label="Browse by category">
-  <a href="{{ shop_url }}{{ facet.facet_qs(sort_key, price_min, price_max) }}"
-     class="tag-chip{% if not active_tag %} tag-chip-active{% endif %}"
-     data-tag-slug=""
-     rel="nofollow">All</a>
-{% for chip in home_chips %}
-  <a href="{{ tag_base }}/tag/{{ chip.slug }}{{ facet.facet_qs(sort_key, price_min, price_max) }}"
-     class="tag-chip{% if active_tag and active_tag.id == chip.id %} tag-chip-active{% endif %}"
-     data-tag-slug="{{ chip.slug }}"
-     rel="nofollow">{{ chip.name }}</a>
-{% endfor %}
-</nav>
-{% endif %}
+{# MPS-24: tag chip strip — shared macro (single source of truth),
+   shown on layout 1 (chips) and 2 (lanes) AND every SERP page. Chip
+   strip + facet left-nav point at the SAME targets; tag_filter.js
+   keeps full navigation for /tag/ hrefs so a chip and its sidebar
+   entry land on the same SERP with the same active state. #}
+{{ facet.chip_strip(shop_url, tag_base, home_chips, active_tag, sort_key, price_min, price_max) }}
 
 {% if show_facet_nav %}
 {# Mobile/tablet: collapsed <details> facets above the grid. #}

make_post_sell/templates/shop_tag.j2

@@ -11,6 +11,11 @@
   <p class="type-body-sm"><a href="{{ shop_url }}" class="shop-theme-link-color">&larr; All products</a></p>
 </section>
 
+{# Chip strip stays on screen on every SERP page so the shopper can
+   hop categories without going back. Same shared macro as the shop
+   home; active_tag highlights the current category. #}
+{{ facet.chip_strip(shop_url, tag_base, home_chips, active_tag, sort_key, price_min, price_max) }}
+
 {# Mobile/tablet: collapsed <details> facets above the grid (CSS hides
    it ≥800px where the sidebar takes over). #}
 {{ facet.details(shop_url, tag_base, sort_options, sort_key, price_min, price_max, facet_tags, active_tag) }}

make_post_sell/tests/test_functional.py

@@ -8947,6 +8947,43 @@ class TestHomeLayoutAndTags(_AuthenticatedBase):
         self.assertIn('tag-detail-header', body)
         self.assertIn('serp-list', body)
 
+    def test_chip_strip_stays_on_tag_detail_serp(self):
+        """Operator: 'leave the chits on screen for all serp pages.'
+        The horizontal chip strip must render on the tag-detail SERP
+        (not just the shop home) so the shopper can hop categories
+        without going back. The current category chip is active."""
+        shop, product = self._make_shop_with_product("chip-serp-shop")
+        self.testapp.post(
+            f"/s/{shop.id}/settings",
+            {
+                "form_section": "home-layout-settings",
+                "home_layout": "2",
+                "home_layout_tag_limit": "5",
+                "home_layout_per_lane_limit": "10",
+                "submit": "Save Home Layout",
+            },
+        )
+        self.testapp.post(
+            f"/p/{product.id}/edit",
+            {
+                "title": product.title,
+                "description": "One. Two. Three. Four. Five. Six.",
+                "price": str(product.price),
+                "visibility": "1",
+                "tags": "Math",
+            },
+        )
+        res = self.testapp.get(f"/s/{shop.id}/tag/math")
+        body = res.body.decode()
+        # The shared chip strip is present on the SERP page.
+        self.assertIn('class="tag-chip-strip"', body)
+        self.assertIn('data-tag-strip', body)
+        # The active category chip is highlighted on the SERP.
+        self.assertIn('tag-chip tag-chip-active', body)
+        # And it still has the facet sidebar (chip strip is additive,
+        # not a replacement).
+        self.assertIn('facet-nav', body)
+
     def test_facet_links_preserve_sort_and_price(self):
         """Operator report: 'switching one breaks it' — changing category
         reset sort/price. Every category link / All link / chip / lane