--- a/layout/base/GeckoRestyleManager.cpp
+++ b/layout/base/GeckoRestyleManager.cpp
@@ -600,24 +600,23 @@ GeckoRestyleManager::UpdateOnlyAnimation
   }
 
   ProcessRestyles(tracker);
 
   transitionManager->SetInAnimationOnlyStyleUpdate(false);
 }
 
 void
-GeckoRestyleManager::PostRestyleEventInternal(bool aForLazyConstruction)
+GeckoRestyleManager::PostRestyleEventInternal()
 {
   // Make sure we're not in a style refresh; if we are, we still have
   // a call to ProcessPendingRestyles coming and there's no need to
   // add ourselves as a refresh observer until then.
-  bool inRefresh = !aForLazyConstruction && mInStyleRefresh;
   nsIPresShell* presShell = PresContext()->PresShell();
-  if (!inRefresh) {
+  if (!mInStyleRefresh) {
     presShell->ObserveStyleFlushes();
   }
 
   // Unconditionally flag our document as needing a flush.  The other
   // option here would be a dedicated boolean to track whether we need
   // to do so (set here and unset in ProcessPendingRestyles).
   presShell->SetNeedStyleFlush();
 }
@@ -645,17 +644,17 @@ GeckoRestyleManager::PostRestyleEvent(El
   // Set mHavePendingNonAnimationRestyles for any restyle that could
   // possibly contain non-animation styles (i.e., those that require us
   // to do an animation-only style flush before processing style changes
   // to ensure correct initialization of CSS transitions).
   if (aRestyleHint & ~eRestyle_AllHintsWithAnimations) {
     mHavePendingNonAnimationRestyles = true;
   }
 
-  PostRestyleEventInternal(false);
+  PostRestyleEventInternal();
 }
 
 void
 GeckoRestyleManager::PostRebuildAllStyleDataEvent(nsChangeHint aExtraHint,
                                                   nsRestyleHint aRestyleHint)
 {
   NS_ASSERTION(!(aExtraHint & nsChangeHint_ReconstructFrame),
                "Should not reconstruct the root of the frame tree.  "
@@ -664,17 +663,17 @@ GeckoRestyleManager::PostRebuildAllStyle
              "PostRebuildAllStyleDataEvent does not handle "
              "eRestyle_SomeDescendants");
 
   mDoRebuildAllStyleData = true;
   mRebuildAllExtraHint |= aExtraHint;
   mRebuildAllRestyleHint |= aRestyleHint;
 
   // Get a restyle event posted if necessary
-  PostRestyleEventInternal(false);
+  PostRestyleEventInternal();
 }
 
 // aContent must be the content for the frame in question, which may be
 // :before/:after content
 /* static */ bool
 GeckoRestyleManager::TryInitiatingTransition(nsPresContext* aPresContext,
                                              nsIContent* aContent,
                                              nsStyleContext* aOldStyleContext,

--- a/layout/base/GeckoRestyleManager.h
+++ b/layout/base/GeckoRestyleManager.h
@@ -52,18 +52,16 @@ protected:
   }
 
 public:
   // Forwarded nsIDocumentObserver method, to handle restyling (and
   // passing the notification to the frame).
   void ContentStateChanged(nsIContent* aContent,
                            EventStates aStateMask);
 
-  void PostRestyleEventForLazyConstruction() { PostRestyleEventInternal(true); }
-
   // Forwarded nsIMutationObserver method, to handle restyling.
   void AttributeWillChange(Element* aElement,
                            int32_t  aNameSpaceID,
                            nsIAtom* aAttribute,
                            int32_t  aModType,
                            const nsAttrValue* aNewValue);
   // Forwarded nsIMutationObserver method, to handle restyling (and
   // passing the notification to the frame).
@@ -110,18 +108,20 @@ public:
    * @param aFrame the root of the subtree to reparent.  Must not be null.
    */
   nsresult ReparentStyleContext(nsIFrame* aFrame);
 
   void ClearSelectors() {
     mPendingRestyles.ClearSelectors();
   }
 
+  void PostRestyleEventForLazyConstruction() { PostRestyleEventInternal(); }
+
 private:
-  void PostRestyleEventInternal(bool aForLazyConstruction);
+  void PostRestyleEventInternal();
 
   // Used when restyling an element with a frame.
   void ComputeAndProcessStyleChange(nsIFrame*              aFrame,
                                     nsChangeHint           aMinChange,
                                     RestyleTracker&        aRestyleTracker,
                                     nsRestyleHint          aRestyleHint,
                                     const RestyleHintData& aRestyleHintData);
 

--- a/layout/base/nsCSSFrameConstructor.cpp
+++ b/layout/base/nsCSSFrameConstructor.cpp
@@ -7408,16 +7408,17 @@ nsCSSFrameConstructor::StyleNewChildRang
 
 void
 nsCSSFrameConstructor::ContentAppended(nsIContent* aContainer,
                                        nsIContent* aFirstNewContent,
                                        bool aAllowLazyConstruction,
                                        TreeMatchContext* aProvidedTreeMatchContext)
 {
   MOZ_ASSERT_IF(aProvidedTreeMatchContext, !aAllowLazyConstruction);
+  MOZ_ASSERT_IF(aAllowLazyConstruction, !RestyleManager()->IsInStyleRefresh());
 
   AUTO_LAYOUT_PHASE_ENTRY_POINT(mPresShell->GetPresContext(), FrameC);
   NS_PRECONDITION(mUpdateCount != 0,
                   "Should be in an update while creating frames");
 
 #ifdef DEBUG
   if (gNoisyContentUpdates) {
     printf("nsCSSFrameConstructor::ContentAppended container=%p "
@@ -7481,21 +7482,16 @@ nsCSSFrameConstructor::ContentAppended(n
       }
       return;
     }
 
     if (aAllowLazyConstruction &&
         MaybeConstructLazily(CONTENTAPPEND, aContainer, aFirstNewContent)) {
       if (isNewlyAddedContentForServo) {
         aContainer->AsElement()->NoteDirtyDescendantsForServo();
-      } else {
-        // Lazy frame construction is done by the restyle flushes, so we need to
-        // ensure a refresh happens.
-        mPresShell->SetNeedStyleFlush();
-        mPresShell->ObserveStyleFlushes();
       }
       return;
     }
   }
 
   // We couldn't construct lazily. Make Servo eagerly traverse the new content.
   if (isNewlyAddedContentForServo) {
     StyleNewChildRange(aFirstNewContent, nullptr);
@@ -7812,16 +7808,19 @@ nsCSSFrameConstructor::ContentInserted(n
 void
 nsCSSFrameConstructor::ContentRangeInserted(nsIContent* aContainer,
                                             nsIContent* aStartChild,
                                             nsIContent* aEndChild,
                                             nsILayoutHistoryState* aFrameState,
                                             bool aAllowLazyConstruction,
                                             TreeMatchContext* aProvidedTreeMatchContext)
 {
+  MOZ_ASSERT_IF(aProvidedTreeMatchContext, !aAllowLazyConstruction);
+  MOZ_ASSERT_IF(aAllowLazyConstruction, !RestyleManager()->IsInStyleRefresh());
+
   AUTO_LAYOUT_PHASE_ENTRY_POINT(mPresShell->GetPresContext(), FrameC);
   NS_PRECONDITION(mUpdateCount != 0,
                   "Should be in an update while creating frames");
 
   NS_PRECONDITION(aStartChild, "must always pass a child");
 
   // XXXldb Do we need to re-resolve style to handle the CSS2 + combinator and
   // the :empty pseudo-class?
@@ -7963,21 +7962,16 @@ nsCSSFrameConstructor::ContentRangeInser
     // Otherwise, we've got parent content. Find its frame.
     NS_ASSERTION(!parentFrame || parentFrame->GetContent() == aContainer ||
                  GetDisplayContentsStyleFor(aContainer), "New XBL code is possibly wrong!");
 
     if (aAllowLazyConstruction &&
         MaybeConstructLazily(CONTENTINSERT, aContainer, aStartChild)) {
       if (isNewlyAddedContentForServo) {
         aContainer->AsElement()->NoteDirtyDescendantsForServo();
-      } else {
-        // Lazy frame construction is done by the restyle flushes, so we need to
-        // ensure a refresh happens.
-        mPresShell->SetNeedStyleFlush();
-        mPresShell->ObserveStyleFlushes();
       }
       return;
     }
   }
 
   // We couldn't construct lazily. Make Servo eagerly traverse the new content.
   if (isNewlyAddedContentForServo) {
     StyleNewChildRange(aStartChild, aEndChild);