diff --git a/dom/base/Document.cpp b/dom/base/Document.cpp
index 3ed1b3a5b312..965bd7f9078c 100644
--- a/dom/base/Document.cpp
+++ b/dom/base/Document.cpp
@@ -16422,47 +16422,33 @@ WindowContext* Document::GetWindowContextForPageUseCounters() const {
   return wc;
 }
 
-void Document::UpdateIntersectionObservations(TimeStamp aNowTime) {
-  if (mIntersectionObservers.IsEmpty()) {
-    return;
-  }
-
-  DOMHighResTimeStamp time = 0;
-  if (nsPIDOMWindowInner* win = GetInnerWindow()) {
-    if (Performance* perf = win->GetPerformance()) {
-      time = perf->TimeStampToDOMHighResForRendering(aNowTime);
+void Document::UpdateIntersections(TimeStamp aNowTime) {
+  if (!mIntersectionObservers.IsEmpty()) {
+    DOMHighResTimeStamp time = 0;
+    if (nsPIDOMWindowInner* win = GetInnerWindow()) {
+      if (Performance* perf = win->GetPerformance()) {
+        time = perf->TimeStampToDOMHighResForRendering(aNowTime);
+      }
     }
-  }
-
-  const auto observers = ToTArray<nsTArray<RefPtr<DOMIntersectionObserver>>>(
-      mIntersectionObservers);
-  for (const auto& observer : observers) {
-    if (observer) {
+    for (DOMIntersectionObserver* observer : mIntersectionObservers) {
       observer->Update(*this, time);
     }
+    Dispatch(NewRunnableMethod("Document::NotifyIntersectionObservers", this,
+                               &Document::NotifyIntersectionObservers));
   }
-}
-
-void Document::ScheduleIntersectionObserverNotification() {
-  if (mIntersectionObservers.IsEmpty()) {
-    return;
-  }
-  MOZ_RELEASE_ASSERT(NS_IsMainThread());
-  nsCOMPtr<nsIRunnable> notification =
-      NewRunnableMethod("Document::NotifyIntersectionObservers", this,
-                        &Document::NotifyIntersectionObservers);
-  Dispatch(notification.forget());
+  EnumerateSubDocuments([aNowTime](Document& aDoc) {
+    aDoc.UpdateIntersections(aNowTime);
+    return CallState::Continue;
+  });
 }
 
 void Document::NotifyIntersectionObservers() {
   const auto observers = ToTArray<nsTArray<RefPtr<DOMIntersectionObserver>>>(
       mIntersectionObservers);
   for (const auto& observer : observers) {
-    if (observer) {
-      // MOZ_KnownLive because the 'observers' array guarantees to keep it
-      // alive.
-      MOZ_KnownLive(observer)->Notify();
-    }
+    // MOZ_KnownLive because the 'observers' array guarantees to keep it
+    // alive.
+    MOZ_KnownLive(observer)->Notify();
   }
 }
 
diff --git a/dom/base/Document.h b/dom/base/Document.h
index 0b0d0ca3d0fd..e919f19be08d 100644
--- a/dom/base/Document.h
+++ b/dom/base/Document.h
@@ -3709,8 +3709,9 @@ class Document : public nsINode,
     return !mIntersectionObservers.IsEmpty();
   }
 
-  void UpdateIntersectionObservations(TimeStamp aNowTime);
-  void ScheduleIntersectionObserverNotification();
+  // Update intersection observers in this document and all
+  // same-process subdocuments.
+  void UpdateIntersections(TimeStamp aNowTime);
   MOZ_CAN_RUN_SCRIPT void NotifyIntersectionObservers();
 
   DOMIntersectionObserver* GetLazyLoadObserver() { return mLazyLoadObserver; }
diff --git a/layout/base/nsRefreshDriver.cpp b/layout/base/nsRefreshDriver.cpp
index 7885d6b8dbe3..0ec39783c265 100644
--- a/layout/base/nsRefreshDriver.cpp
+++ b/layout/base/nsRefreshDriver.cpp
@@ -2228,23 +2228,7 @@ void nsRefreshDriver::RunFullscreenSteps() {
 
 void nsRefreshDriver::UpdateIntersectionObservations(TimeStamp aNowTime) {
   AUTO_PROFILER_LABEL_RELEVANT_FOR_JS("Compute intersections", LAYOUT);
-
-  AutoTArray<RefPtr<Document>, 32> documents;
-
-  if (mPresContext->Document()->HasIntersectionObservers()) {
-    documents.AppendElement(mPresContext->Document());
-  }
-
-  mPresContext->Document()->CollectDescendantDocuments(
-      documents, [](const Document* document) -> bool {
-        return document->HasIntersectionObservers();
-      });
-
-  for (const auto& doc : documents) {
-    doc->UpdateIntersectionObservations(aNowTime);
-    doc->ScheduleIntersectionObserverNotification();
-  }
-
+  mPresContext->Document()->UpdateIntersections(aNowTime);
   mNeedToUpdateIntersectionObservations = false;
 }