🐛 [RUM-13741] Fix sibling uniqueness check for elements in DocumentFragment

packages/rum-core/src/domain/getSelectorFromElement.spec.ts

@@ -219,4 +219,23 @@ describe('isSelectorUniqueAmongSiblings', () => {
     `)
     expect(isSelectorUniqueAmongSiblings(element, 'DIV', 'HR')).toBeTrue()
   })
+
+  it('returns false when element is in DocumentFragment with matching siblings', () => {
+    const fragment = document.createDocumentFragment()
+    const div1 = document.createElement('div')
+    const div2 = document.createElement('div')
+    fragment.appendChild(div1)
+    fragment.appendChild(div2)
+
+    // The function should return false because div2 matches 'DIV' selector
+    expect(isSelectorUniqueAmongSiblings(div1, 'DIV', undefined)).toBeFalse()
+  })
+
+  it('returns true when element is in DocumentFragment with no matching siblings', () => {
+    const fragment = document.createDocumentFragment()
+    const div = document.createElement('div')
+    fragment.appendChild(div)
+
+    expect(isSelectorUniqueAmongSiblings(div, 'DIV', undefined)).toBeTrue()
+  })
 })

packages/rum-core/src/domain/getSelectorFromElement.ts

@@ -249,10 +249,22 @@ export function isSelectorUniqueAmongSiblings(
     isSiblingMatching = (sibling) => sibling.querySelector(scopedSelector) !== null
   }
 
-  const parent = currentElement.parentElement!
-  let sibling = parent.firstElementChild
+  // Check siblings by iterating directly through previousElementSibling and nextElementSibling.
+  // This works even when parentElement is null (e.g., when parent is a DocumentFragment).
+
+  // Check previous siblings
+  let sibling = currentElement.previousElementSibling
+  while (sibling) {
+    if (isSiblingMatching(sibling)) {
+      return false
+    }
+    sibling = sibling.previousElementSibling
+  }
+
+  // Check next siblings
+  sibling = currentElement.nextElementSibling
   while (sibling) {
-    if (sibling !== currentElement && isSiblingMatching(sibling)) {
+    if (isSiblingMatching(sibling)) {
       return false
     }
     sibling = sibling.nextElementSibling