Bug 1440537 - Don't propagate flushes across docgroup boundaries. r=bzbarsky

We don't need to flush layout in the parent document if the parent and child
documents can't observe each other.

This will also match our behavior in a Fission world.

I'm not attached to the name of the function, better ideas welcome.

Differential Revision: https://phabricator.services.mozilla.com/D28217

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;
@@ -3162,10 +3162,8 @@ mozilla::dom::DocGroup* Document::GetDocGroup() const {
     }
     // XXX: Check that the TabGroup is correct as well!
   }
-#endif
-
-  return mDocGroup;
 }
+#endif
 
 nsresult Document::Dispatch(TaskCategory aCategory,
                             already_AddRefed<nsIRunnable>&& aRunnable) {
@@ -7334,7 +7332,8 @@ 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 (mParentDocument && IsSafeToFlush()) {
+  if (StyleOrLayoutObservablyDependsOnParentDocumentLayout() &&
+      IsSafeToFlush()) {
     mozilla::ChangesToFlush parentFlush = aFlush;
     if (flushType >= FlushType::Style) {
       parentFlush.mFlushType = std::max(FlushType::Layout, flushType);

dom/base/Document.h

@@ -2406,7 +2406,6 @@ 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
@@ -3497,7 +3496,30 @@ class Document : public nsINode,
   void ReportHasScrollLinkedEffect();
   bool HasScrollLinkedEffect() const { return mHasScrollLinkedEffect; }
 
-  DocGroup* GetDocGroup() const;
+#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();
+  }
 
   void AddIntersectionObserver(DOMIntersectionObserver* aObserver) {
     MOZ_ASSERT(!mIntersectionObservers.Contains(aObserver),

dom/base/nsGlobalWindowOuter.cpp

@@ -7366,17 +7366,13 @@ void nsGlobalWindowOuter::FlushPendingNotifications(FlushType aType) {
 }
 
 void nsGlobalWindowOuter::EnsureSizeAndPositionUpToDate() {
-  // 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()) {
+  // 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);
   }
-  }
 }
 
 already_AddRefed<nsISupports> nsGlobalWindowOuter::SaveWindowState() {

layout/style/nsComputedDOMStyle.cpp

@@ -785,7 +785,8 @@ 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 (Document* parentDocument = doc->GetParentDocument()) {
+  while (doc->StyleOrLayoutObservablyDependsOnParentDocumentLayout()) {
+    Document* parentDocument = doc->GetParentDocument();
     Element* element = parentDocument->FindContentForSubDocument(doc);
     if (ElementNeedsRestyle(element, nullptr)) {
       return true;