1 files changed, 57 insertions, 5 deletions
diff --git a/src/content/dependencies/generateStickyHeadingContainer.js b/src/content/dependencies/generateStickyHeadingContainer.js
index 7f271715..ab607a4f 100644
--- a/src/content/dependencies/generateStickyHeadingContainer.js
+++ b/src/content/dependencies/generateStickyHeadingContainer.js
@@ -28,11 +28,63 @@ export default {
             html.tag('div', {class: 'content-sticky-heading-cover'},
               {[html.onlyIfContent]: true},
 
-              // TODO: We shouldn't need to do an isBlank check here,
-              // but a live blank value doesn't have a slot functions, so.
-              (html.isBlank(slots.cover)
-                ? html.blank()
-                : slots.cover.slot('mode', 'thumbnail')))),
+              (() => {
+                if (html.isBlank(slots.cover)) {
+                  return html.blank();
+                }
+
+                // Try very hard to set the cover's 'mode' slot to 'thumbnail'
+                // and its 'details' slot to html.blank().
+                let setMode = false;
+                let setDetails = false;
+                let cover = slots.cover;
+                while (true) {
+                  if (!cover) {
+                    return html.blank();
+                  }
+
+                  if (!(cover instanceof html.Template)) {
+                    return cover;
+                  }
+
+                  // The cover from `slots` is already cloned (since it's
+                  // mutable), but later ones are not, and for extremely scary
+                  // content function infrastructure reasons, it is possible
+                  // for the identity of the content of the clone-template
+                  // to be the same as the cloned template.
+                  if (cover !== slots.cover) {
+                    cover = cover.clone();
+                  }
+
+                  if (!setMode) {
+                    try {
+                      cover.setSlot('mode', 'thumbnail');
+                      setMode = true;
+                    } catch {
+                      // No mode slot, or it doesn't allow 'thumbnail'.
+                    }
+                  }
+
+                  if (!setDetails) {
+                    try {
+                      cover.setSlot('details', html.blank());
+                      setDetails = true;
+                    } catch {
+                      // No details slot, or it doesn't allow html.blank().
+                      // We're setting a blank instead of null because null is
+                      // always allowed, and can carry a different semantic
+                      // meaning, like "put something else here by default
+                      // instead please".
+                    }
+                  }
+
+                  if (setMode && setDetails) {
+                    return cover;
+                  }
+
+                  cover = cover.content;
+                }
+              })())),
         ]),
 
         html.tag('div', {class: 'content-sticky-subheading-row'},