Fix keyboard access for scrollable regions created by notebook outputs

src/pydata_sphinx_theme/assets/scripts/pydata-sphinx-theme.js

@@ -693,10 +693,11 @@ function setupMobileSidebarKeyboardHandlers() {
 }
 
 /**
- * When the page loads or the window resizes check code blocks and Jupyter
- * notebook outputs, and if they have scrollable overflow, set tabIndex = 0.
+ * When the page loads, or the window resizes, or descendant nodes are added or
+ * removed from the main element, check all code blocks and Jupyter notebook
+ * outputs, and for each one that has scrollable overflow, set tabIndex = 0.
  */
-function setupLiteralBlockTabStops() {
+function addTabStopsToScrollableElements() {
   const updateTabStops = () => {
     document
       .querySelectorAll(
@@ -712,7 +713,25 @@ function setupLiteralBlockTabStops() {
             : -1;
       });
   };
-  window.addEventListener("resize", debounce(updateTabStops, 300));
+  const debouncedUpdateTabStops = debounce(updateTabStops, 300);
+
+  // On window resize
+  window.addEventListener("resize", debouncedUpdateTabStops);
+
+  // The following MutationObserver is for ipywidgets, which take some time to
+  // finish loading and rendering on the page (so even after the "load" event is
+  // fired, they still have not finished rendering). Would be nice to replace
+  // the MutationObserver if there is a way to hook into the ipywidgets code to
+  // know when it is done.
+  const mainObserver = new MutationObserver(debouncedUpdateTabStops);
+
+  // On descendant nodes added/removed from main element
+  mainObserver.observe(document.getElementById("main-content"), {
+    subtree: true,
+    childList: true,
+  });
+
+  // On page load
   updateTabStops();
 }
 function debounce(callback, wait) {
@@ -736,4 +755,7 @@ documentReady(setupSearchButtons);
 documentReady(initRTDObserver);
 documentReady(setupMobileSidebarKeyboardHandlers);
 documentReady(fixMoreLinksInMobileSidebar);
-documentReady(setupLiteralBlockTabStops);
+
+// Use load event because determining whether an element has scrollable content
+// depends on stylesheets (which come after DOMContentLoaded)
+window.addEventListener("load", addTabStopsToScrollableElements);

src/pydata_sphinx_theme/assets/styles/extensions/_notebooks.scss

@@ -12,6 +12,11 @@
 html div.rendered_html,
 // NBsphinx ipywidgets output selector
 html .jp-RenderedHTMLCommon {
+  // Add some margin around the element box for the focus ring. Otherwise the
+  // focus ring gets clipped because the containing elements have `overflow:
+  // hidden` applied to them (via the `.lm-Widget` selector)
+  margin: $focus-ring-width;
+
   table {
     table-layout: auto;
   }