With much tribulation, accept title paragraphs into the case study teaser fold

agaric/templates/field/field--paragraph--field-subtitle--preview.html.twig

@@ -0,0 +1,6 @@
+{# Overrides field--field-subtitle.html.twig for case study teasers (which use
+   the title paragraph 'preview' view mode). #}
+
+{% for item in items %}
+  {{ item.content}}
+{% endfor %}

agaric/templates/layout/node--case-study--teaser.html.twig

@@ -1,23 +1,27 @@
-{# Somewhat ironically, we take away the ability for case studies to be
-   represented by their title paragraph.  Because we *don't* use the title
-   paragraph for the main listings on the front page and the /work page (both of
-   which use the tile view mode, which uses only set regular fields), the use of
-   the title paragraph for search results (and anywhere else we use teasers)
-   would be a surprise— the summary field, while also not used on the listing
-   pages, is at least consistently used across multiple content types.
-
-   See agaric/templates/layout/node--teaser.html.twig for variables.
-#}
+{# See agaric/templates/layout/node--teaser.html.twig for variables. #}
 
 <a class="teaser media content" href="{{ url }}">
 
 <div class="media-left">
+
+{% set has_title_paragraph = false %}
+{% if content.field_body_paragraph %}
+  {% for i, paragraph in content.field_body_paragraph if i|first != '#' %}
+    {% if content.field_body_paragraph['#items'][i].entity.type.0.entity.id == 'title' %}
+      {% set has_title_paragraph = true %}
+      {{ paragraph }}
+    {% endif %}
+  {% endfor %}
+{% endif %}
+
+{% if not has_title_paragraph %}
   {{ content.field_image }}
   {% if node.bundle == 'case_study' and content.field_case_study_subtitle is not empty %}
     <p class="byline is-mono">
       {{ content.field_case_study_subtitle }}
     </p>
   {% endif %}
+{% endif %}
 </div>
 <div class="media-content content">
   <h3 class="title is-5">{{ label }}</h3>

agaric/templates/paragraph/paragraph--title--preview.html.twig

@@ -0,0 +1,37 @@
+{#
+/**
+ * The paragraph preview view mode is used in the node teaser view mode.
+ *
+ * @see agaric/templates/paragraph/paragraph--title.html.twig for variables.
+ */
+#}
+{%
+  set classes = [
+    'hero',
+    not paragraph.isPublished() ? 'paragraph--unpublished'
+  ]
+%}
+{% if paragraph.field_style_color is not empty %}
+  {% set classes = classes|merge(['is-'~paragraph.field_style_color.value]) %}
+{% endif %}
+{% if paragraph.field_style_titlebar.value == false %}
+  {% set classes = classes|merge(['hero-project']) %}
+{% endif %}
+{% if paragraph.field_style_classes is not empty %}
+  {% set styles=[] %}
+  {% for style in paragraph.field_style_classes %}
+    {% set styles = styles|merge([style.value]) %}
+  {% endfor %}
+  {% set classes = classes|merge(styles) %}
+{% endif %}
+
+{% if paragraph.field_style_titlebar.value == true and paragraph.field_image is not empty %}
+  {% set attributes = attributes.setAttribute('data-headerbg', 'transparent').setAttribute('style', 'background-image: url('~file_url(paragraph.field_image.entity.uri.value)~')') %}
+{% endif %}
+
+{% block paragraph %}
+    {% block content %}
+      {{ content.field_image }}
+      <p class="byline is-mono">{{ content.field_subtitle }}</p>
+    {% endblock %}
+{% endblock paragraph %}