--- a/layout/base/PresShell.cpp
+++ b/layout/base/PresShell.cpp
@@ -737,16 +737,17 @@ nsIPresShell::nsIPresShell()
 #endif
 #ifdef DEBUG
     , mDrawEventTargetFrame(nullptr)
 #endif
     , mPaintCount(0)
     , mAutoWeakFrames(nullptr)
     , mCanvasBackgroundColor(NS_RGBA(0,0,0,0))
     , mSelectionFlags(0)
+    , mChangeNestCount(0)
     , mRenderFlags(0)
     , mDidInitialize(false)
     , mIsDestroying(false)
     , mIsReflowing(false)
     , mPaintingSuppressed(false)
     , mIsActive(false)
     , mFrozen(false)
     , mIsFirstPaint(false)
@@ -782,17 +783,16 @@ PresShell::PresShell()
 #endif
   , mMouseLocation(NS_UNCONSTRAINEDSIZE, NS_UNCONSTRAINEDSIZE)
   , mCurrentEventFrame(nullptr)
   , mFirstCallbackEventRequest(nullptr)
   , mLastCallbackEventRequest(nullptr)
   , mLastReflowStart(0.0)
   , mLastAnchorScrollPositionY(0)
   , mAPZFocusSequenceNumber(0)
-  , mChangeNestCount(0)
   , mDocumentLoading(false)
   , mIgnoreFrameDestruction(false)
   , mHaveShutDown(false)
   , mLastRootReflowHadUnconstrainedBSize(false)
   , mNoDelayedMouseEvents(false)
   , mNoDelayedKeyEvents(false)
   , mIsDocumentGone(false)
   , mShouldUnsuppressPainting(false)
@@ -4033,35 +4033,33 @@ PresShell::HandlePostedReflowCallbacks(b
    FlushType flushType =
      aInterruptible ? FlushType::InterruptibleLayout : FlushType::Layout;
    if (shouldFlush && !mIsDestroying) {
      FlushPendingNotifications(flushType);
    }
 }
 
 bool
-PresShell::IsSafeToFlush() const
+nsIPresShell::IsSafeToFlush() const
 {
   // Not safe if we are reflowing or in the middle of frame construction
-  bool isSafeToFlush = !mIsReflowing &&
-                         !mChangeNestCount;
-
-  if (isSafeToFlush) {
+  if (mIsReflowing || mChangeNestCount) {
+    return false;
+  }
+
     // Not safe if we are painting
-    nsViewManager* viewManager = GetViewManager();
-    if (viewManager) {
-      bool isPainting = false;
-      viewManager->IsPainting(isPainting);
-      if (isPainting) {
-        isSafeToFlush = false;
-      }
-    }
-  }
-
-  return isSafeToFlush;
+  if (nsViewManager* viewManager = GetViewManager()) {
+    bool isPainting = false;
+    viewManager->IsPainting(isPainting);
+    if (isPainting) {
+      return false;
+    }
+  }
+
+  return true;
 }
 
 
 void
 PresShell::DoFlushPendingNotifications(FlushType aType)
 {
   // by default, flush animations if aType >= FlushType::Style
   mozilla::ChangesToFlush flush(aType, aType >= FlushType::Style);

--- a/layout/base/PresShell.h
+++ b/layout/base/PresShell.h
@@ -123,17 +123,16 @@ public:
   virtual nsCanvasFrame* GetCanvasFrame() const override;
 
   virtual void FrameNeedsReflow(nsIFrame *aFrame, IntrinsicDirty aIntrinsicDirty,
                                 nsFrameState aBitToAdd,
                                 ReflowRootHandling aRootHandling =
                                   eInferFromBitToAdd) override;
   virtual void FrameNeedsToContinueReflow(nsIFrame *aFrame) override;
   virtual void CancelAllPendingReflows() override;
-  virtual bool IsSafeToFlush() const override;
   virtual void DoFlushPendingNotifications(mozilla::FlushType aType) override;
   virtual void DoFlushPendingNotifications(mozilla::ChangesToFlush aType) override;
   virtual void DestroyFramesForAndRestyle(mozilla::dom::Element* aElement) override;
 
   /**
    * Post a callback that should be handled after reflow has finished.
    */
   virtual nsresult PostReflowCallback(nsIReflowCallback* aCallback) override;
@@ -852,21 +851,16 @@ protected:
   // when target of pointer event was deleted during executing user handlers.
   nsCOMPtr<nsIContent>      mPointerEventTarget;
 
   // The focus sequence number of the last processed input event
   uint64_t                  mAPZFocusSequenceNumber;
   // The focus information needed for async keyboard scrolling
   FocusTarget               mAPZFocusTarget;
 
-  // This is used to protect ourselves from triggering reflow while in the
-  // middle of frame construction and the like... it really shouldn't be
-  // needed, one hopes, but it is for now.
-  uint16_t                  mChangeNestCount;
-
   bool                      mDocumentLoading : 1;
   bool                      mIgnoreFrameDestruction : 1;
   bool                      mHaveShutDown : 1;
   bool                      mLastRootReflowHadUnconstrainedBSize : 1;
   bool                      mNoDelayedMouseEvents : 1;
   bool                      mNoDelayedKeyEvents : 1;
 
   // We've been disconnected from the document.  We will refuse to paint the

--- a/layout/base/nsIPresShell.h
+++ b/layout/base/nsIPresShell.h
@@ -536,21 +536,19 @@ public:
                            nsRestyleHint aHint);
 
   // ShadowRoot has APIs that can change styles so we only
   // want to restyle elements in the ShadowRoot and not the whole
   // document.
   virtual void RecordShadowStyleChange(mozilla::dom::ShadowRoot* aShadowRoot) = 0;
 
   /**
-   * Determine if it is safe to flush all pending notifications
-   * @param aIsSafeToFlush true if it is safe, false otherwise.
-   *
+   * Determine if it is safe to flush all pending notifications.
    */
-  virtual bool IsSafeToFlush() const = 0;
+  bool IsSafeToFlush() const;
 
   /**
    * Flush pending notifications of the type specified.  This method
    * will not affect the content model; it'll just affect style and
    * frames. Callers that actually want up-to-date presentation (other
    * than the document itself) should probably be calling
    * nsIDocument::FlushPendingNotifications.
    *
@@ -1703,16 +1701,21 @@ protected:
   nscolor                   mCanvasBackgroundColor;
 
   // Used to force allocation and rendering of proportionally more or
   // less pixels in both dimensions.
   mozilla::Maybe<float>     mResolution;
 
   int16_t                   mSelectionFlags;
 
+  // This is used to protect ourselves from triggering reflow while in the
+  // middle of frame construction and the like... it really shouldn't be
+  // needed, one hopes, but it is for now.
+  uint16_t                  mChangeNestCount;
+
   // Flags controlling how our document is rendered.  These persist
   // between paints and so are tied with retained layer pixels.
   // PresShell flushes retained layers when the rendering state
   // changes in a way that prevents us from being able to (usefully)
   // re-use old pixels.
   RenderFlags               mRenderFlags;
   bool                      mDidInitialize : 1;
   bool                      mIsDestroying : 1;