diff --git a/dom/base/Document.cpp b/dom/base/Document.cpp
index 742df2c96d4f..cac6d1db11ed 100644
--- a/dom/base/Document.cpp
+++ b/dom/base/Document.cpp
@@ -3149,8 +3149,8 @@ void Document::SetPrincipals(nsIPrincipal* aNewPrincipal,
 #endif
 }
 
+mozilla::dom::DocGroup* Document::GetDocGroup() const {
 #ifdef DEBUG
-void Document::AssertDocGroupMatchesKey() const {
   // Sanity check that we have an up-to-date and accurate docgroup
   if (mDocGroup) {
     nsAutoCString docGroupKey;
@@ -3162,9 +3162,11 @@ void Document::AssertDocGroupMatchesKey() const {
     }
     // XXX: Check that the TabGroup is correct as well!
   }
-}
 #endif
 
+  return mDocGroup;
+}
+
 nsresult Document::Dispatch(TaskCategory aCategory,
                             already_AddRefed<nsIRunnable>&& aRunnable) {
   // Note that this method may be called off the main thread.
@@ -7332,8 +7334,7 @@ void Document::FlushPendingNotifications(mozilla::ChangesToFlush aFlush) {
   // affect style, we need to promote a style flush on ourself to a
   // layout flush on our parent, since we need our container to be the
   // correct size to determine the correct style.
-  if (StyleOrLayoutObservablyDependsOnParentDocumentLayout() &&
-      IsSafeToFlush()) {
+  if (mParentDocument && IsSafeToFlush()) {
     mozilla::ChangesToFlush parentFlush = aFlush;
     if (flushType >= FlushType::Style) {
       parentFlush.mFlushType = std::max(FlushType::Layout, flushType);
diff --git a/dom/base/Document.h b/dom/base/Document.h
index 837dae91ceaa..2dcffdacd122 100644
--- a/dom/base/Document.h
+++ b/dom/base/Document.h
@@ -2406,6 +2406,7 @@ class Document : public nsINode,
                                           nsAtom* aAttrName,
                                           const nsAString& aAttrValue) const;
 
+
   /**
    * To batch DOMSubtreeModified, document needs to be informed when
    * a mutation event might be dispatched, even if the event isn't actually
@@ -3496,30 +3497,7 @@ class Document : public nsINode,
   void ReportHasScrollLinkedEffect();
   bool HasScrollLinkedEffect() const { return mHasScrollLinkedEffect; }
 
-#ifdef DEBUG
-  void AssertDocGroupMatchesKey() const;
-#endif
-
-  DocGroup* GetDocGroup() const {
-#ifdef DEBUG
-    AssertDocGroupMatchesKey();
-#endif
-    return mDocGroup;
-  }
-
-  /**
-   * If we're a sub-document, the parent document's layout can affect our style
-   * and layout (due to the viewport size, viewport units, media queries...).
-   *
-   * This function returns true if our parent document and our child document
-   * can observe each other. If they cannot, then we don't need to synchronously
-   * update the parent document layout every time the child document may need
-   * up-to-date layout information.
-   */
-  bool StyleOrLayoutObservablyDependsOnParentDocumentLayout() const {
-    return GetParentDocument() &&
-           GetDocGroup() == GetParentDocument()->GetDocGroup();
-  }
+  DocGroup* GetDocGroup() const;
 
   void AddIntersectionObserver(DOMIntersectionObserver* aObserver) {
     MOZ_ASSERT(!mIntersectionObservers.Contains(aObserver),
diff --git a/dom/base/nsGlobalWindowOuter.cpp b/dom/base/nsGlobalWindowOuter.cpp
index ac52bd57bd61..88d80391eff6 100644
--- a/dom/base/nsGlobalWindowOuter.cpp
+++ b/dom/base/nsGlobalWindowOuter.cpp
@@ -7366,12 +7366,16 @@ void nsGlobalWindowOuter::FlushPendingNotifications(FlushType aType) {
 }
 
 void nsGlobalWindowOuter::EnsureSizeAndPositionUpToDate() {
-  // If we're a subframe, make sure our size is up to date.  Make sure to go
-  // through the document chain rather than the window chain to not flush on
-  // detached iframes, see bug 1545516.
-  if (mDoc && mDoc->StyleOrLayoutObservablyDependsOnParentDocumentLayout()) {
-    RefPtr<Document> parent = mDoc->GetParentDocument();
-    parent->FlushPendingNotifications(FlushType::Layout);
+  // If we're a subframe, make sure our size is up to date.  It's OK that this
+  // crosses the content/chrome boundary, since chrome can have pending reflows
+  // too.
+  //
+  // Make sure to go through the document chain rather than the window chain to
+  // not flush on detached iframes, see bug 1545516.
+  if (mDoc) {
+    if (RefPtr<Document> parent = mDoc->GetParentDocument()) {
+      parent->FlushPendingNotifications(FlushType::Layout);
+    }
   }
 }
 
diff --git a/layout/style/nsComputedDOMStyle.cpp b/layout/style/nsComputedDOMStyle.cpp
index 2d3da636a37c..fb644a64db73 100644
--- a/layout/style/nsComputedDOMStyle.cpp
+++ b/layout/style/nsComputedDOMStyle.cpp
@@ -785,8 +785,7 @@ bool nsComputedDOMStyle::NeedsToFlush() const {
   Document* doc = mElement->OwnerDoc();
   // If parent document is there, also needs to check if there is some change
   // that needs to flush this document (e.g. size change for iframe).
-  while (doc->StyleOrLayoutObservablyDependsOnParentDocumentLayout()) {
-    Document* parentDocument = doc->GetParentDocument();
+  while (Document* parentDocument = doc->GetParentDocument()) {
     Element* element = parentDocument->FindContentForSubDocument(doc);
     if (ElementNeedsRestyle(element, nullptr)) {
       return true;