Fix types

index.js

@@ -160,17 +160,31 @@ function isFocusable(element) {
   )
     return false
   // Descends from closed details element
-  if (
-    element.closest("details:not([open])") != null &&
-    (element.tagName !== "SUMMARY" ||
-      element.parentElement.parentElement.closest("details:not([open])") !=
-        null)
-  )
-    return false
+  if (hasClosedDetailsAncestor(element)) return false
 
   return true
 }
 
+/**
+ * Tests whether the element is contained within a closed `details` element.
+ *
+ * `summary` elements are excluded (return value `false`) if they are the summary of the top-most
+ * closed `details` element.
+ *
+ * @param {Element} element
+ * @returns {boolean} - True if the element is hidden because of descending from closed `details`
+ */
+function hasClosedDetailsAncestor(element) {
+  if (element.parentElement == null) return false
+
+  const parentElement = element.parentElement
+  if (element.tagName === "SUMMARY") {
+    return hasClosedDetailsAncestor(parentElement)
+  } else {
+    return parentElement.closest("details:not([open])") != null
+  }
+}
+
 /**
  * Get all candidates for receiving focus when moving from the active element in the given direction.
  *