Put each code fold inside its language tab

.gitignore

@@ -9,6 +9,7 @@
 # Source HTML partials we intentionally track (not build output).
 !qwt-reveal-toggle.html
 !theme-picker.html
+!tabset-code-in-tabs.html
 !anchor-landing.html
 !watch-anchors.html
 !table-roles.html

_quarto-website.yml

@@ -200,6 +200,7 @@ format:
     # Replaces Quarto's two-way toggle with a Light / Off-white / Parchment / Dark dropdown.
     include-in-header: theme-picker.html
     include-after-body:
+      - tabset-code-in-tabs.html
       - anchor-landing.html
       - watch-anchors.html
       - table-roles.html
@@ -235,6 +236,7 @@ format:
     search: false # decks duplicate the HTML pages, so the search indexes only the HTML format
     theme: [simple, styles-reveal.scss]
     include-in-header: qwt-reveal-toggle.html
+    include-after-body: tabset-code-in-tabs.html
     slide-number: true
     chalkboard: false
     preview-links: auto

tabset-code-in-tabs.html

@@ -0,0 +1,45 @@
+<script>
+// Put each folded code cell inside the tab it belongs to.
+// Quarto hoists the `details.code-fold` of a cross-referenced figure out of the figure,
+// so an R and a Python tab end up with their folds stacked above the whole figure.
+// For every language tabset, take the run of folds immediately before it (or before the figure
+// that holds it), match each fold to a tab by language, and move it to the top of that tab.
+// The selectors cover both the website tabsets (Bootstrap) and the slide tabsets (tabby).
+document.querySelectorAll("div.panel-tabset").forEach(function (tabset) {
+  var anchor = tabset.closest("div.quarto-float") || tabset;
+  var panes = Array.prototype.slice.call(tabset.querySelectorAll(":scope > .tab-content > div"));
+  var tabs = Array.prototype.slice.call(tabset.querySelectorAll(":scope > ul > li > a"));
+  var folds = [];
+  var node = anchor.previousElementSibling;
+  while (node && node.matches("details.code-fold")) {
+    folds.unshift(node);
+    node = node.previousElementSibling;
+  }
+  if (folds.length === 0 || folds.length !== panes.length || tabs.length !== panes.length) {
+    return;
+  }
+  var language = function (fold) {
+    var code = fold.querySelector("pre.sourceCode");
+    var found = code && Array.prototype.find.call(code.classList, function (name) {
+      return name !== "sourceCode" && name !== "downlit" && name !== "code-with-copy";
+    });
+    return found ? found.toLowerCase() : "";
+  };
+  var used = {};
+  var unused = function () {
+    return tabs.findIndex(function (tab, j) {
+      return !used[j];
+    });
+  };
+  folds.forEach(function (fold) {
+    var index = tabs.findIndex(function (tab, j) {
+      return !used[j] && tab.textContent.trim().toLowerCase() === language(fold);
+    });
+    if (index < 0) {
+      index = unused();
+    }
+    used[index] = true;
+    panes[index].insertBefore(fold, panes[index].firstChild);
+  });
+});
+</script>