Bug 1426649: Stop tracking DOM changes from painting. r=dbaron

I'm pretty sure this is not a problem now, since we don't mutate the DOM from
painting, and we don't have legacy extensions anymore.

Just to confirm, I did a try run with a RELEASE_ASSERT(!CheckDOMModified()), and
it passed.

MozReview-Commit-ID: HTekD8tsz9v

--HG--
extra : rebase_source : 3ae51ff73b921fd720bd09ae9e294f2d85c71644

layout/base/nsCSSFrameConstructor.cpp

@@ -8916,12 +8916,6 @@ nsCSSFrameConstructor::BeginUpdate() {
   NS_ASSERTION(!nsContentUtils::IsSafeToRunScript(),
                "Someone forgot a script blocker");
 
-  nsRootPresContext* rootPresContext =
-    mPresShell->GetPresContext()->GetRootPresContext();
-  if (rootPresContext) {
-    rootPresContext->IncrementDOMGeneration();
-  }
-
 #ifdef DEBUG
   ++mUpdateCount;
 #endif

layout/base/nsPresContext.cpp

@@ -3176,8 +3176,7 @@ nsPresContext::RebuildFontFeatureValues()
 
 nsRootPresContext::nsRootPresContext(nsIDocument* aDocument,
                                      nsPresContextType aType)
-  : nsPresContext(aDocument, aType),
-    mDOMGeneration(0)
+  : nsPresContext(aDocument, aType)
 {
 }
 

layout/base/nsPresContext.h

@@ -1609,18 +1609,6 @@ public:
 
   virtual bool IsRoot() override { return true; }
 
-  /**
-   * Increment DOM-modification generation counter to indicate that
-   * the DOM has changed in a way that might lead to style changes/
-   * reflows/frame creation and destruction.
-   */
-  void IncrementDOMGeneration() { mDOMGeneration++; }
-
-  /**
-   * Get the current DOM generation counter.
-   */
-  uint32_t GetDOMGeneration() { return mDOMGeneration; }
-
   /**
    * Add a runnable that will get called before the next paint. They will get
    * run eventually even if painting doesn't happen. They might run well before
@@ -1674,7 +1662,6 @@ protected:
   nsTHashtable<nsRefPtrHashKey<nsIContent> > mRegisteredPlugins;
   nsTArray<nsCOMPtr<nsIRunnable> > mWillPaintObservers;
   nsRevocableEventPtr<RunWillPaintObservers> mWillPaintFallbackEvent;
-  uint32_t mDOMGeneration;
 };
 
 #ifdef MOZ_REFLOW_PERF

layout/painting/FrameLayerBuilder.cpp

@@ -121,7 +121,6 @@ FrameLayerBuilder::FrameLayerBuilder()
   : mRetainingManager(nullptr)
   , mContainingPaintedLayer(nullptr)
   , mInactiveLayerClip(nullptr)
-  , mDetectedDOMModification(false)
   , mInvalidateAllLayers(false)
   , mInLayerTreeCompressionMode(false)
   , mIsInactiveLayerManager(false)
@@ -1791,9 +1790,6 @@ FrameLayerBuilder::Init(nsDisplayListBuilder* aBuilder, LayerManager* aManager,
 {
   mDisplayListBuilder = aBuilder;
   mRootPresContext = aBuilder->RootReferenceFrame()->PresContext()->GetRootPresContext();
-  if (mRootPresContext) {
-    mInitialDOMGeneration = mRootPresContext->GetDOMGeneration();
-  }
   mContainingPaintedLayer = aLayerData;
   mIsInactiveLayerManager = aIsInactiveLayerManager;
   mInactiveLayerClip = aInactiveLayerClip;
@@ -6033,9 +6029,6 @@ FrameLayerBuilder::PaintItems(nsTArray<ClippedDisplayItem>& aItems,
         cdi->mItem->Paint(aBuilder, aContext);
       }
     }
-
-    if (CheckDOMModified())
-      break;
   }
 
   if (currentClipIsSetInContext) {
@@ -6116,9 +6109,6 @@ FrameLayerBuilder::DrawPaintedLayer(PaintedLayer* aLayer,
   FrameLayerBuilder *layerBuilder = aLayer->Manager()->GetLayerBuilder();
   NS_ASSERTION(layerBuilder, "Unexpectedly null layer builder!");
 
-  if (layerBuilder->CheckDOMModified())
-    return;
-
   PaintedLayerItemsEntry* entry = layerBuilder->mPaintedLayerItems.GetEntry(aLayer);
   NS_ASSERTION(entry, "We shouldn't be drawing into a layer with no items!");
   if (!entry->mContainerLayerFrame) {
@@ -6234,24 +6224,6 @@ FrameLayerBuilder::DrawPaintedLayer(PaintedLayer* aLayer,
   }
 }
 
-bool
-FrameLayerBuilder::CheckDOMModified()
-{
-  if (!mRootPresContext ||
-      mInitialDOMGeneration == mRootPresContext->GetDOMGeneration())
-    return false;
-  if (mDetectedDOMModification) {
-    // Don't spam the console with extra warnings
-    return true;
-  }
-  mDetectedDOMModification = true;
-  // Painting is not going to complete properly. There's not much
-  // we can do here though. Invalidating the window to get another repaint
-  // is likely to lead to an infinite repaint loop.
-  NS_WARNING("Detected DOM modification during paint, bailing out!");
-  return true;
-}
-
 /* static */ void
 FrameLayerBuilder::DumpRetainedLayerTree(LayerManager* aManager, std::stringstream& aStream, bool aDumpHtml)
 {

layout/painting/FrameLayerBuilder.h

@@ -745,13 +745,6 @@ public:
   void ComputeGeometryChangeForItem(DisplayItemData* aData);
 
 protected:
-  /**
-   * Returns true if the DOM has been modified since we started painting,
-   * in which case we should bail out and not paint anymore. This should
-   * never happen, but plugins can trigger it in some cases.
-   */
-  bool CheckDOMModified();
-
   /**
    * The layer manager belonging to the widget that is being retained
    * across paints.
@@ -784,15 +777,6 @@ protected:
    */
   const DisplayItemClip*              mInactiveLayerClip;
 
-  /**
-   * Saved generation counter so we can detect DOM changes.
-   */
-  uint32_t                            mInitialDOMGeneration;
-  /**
-   * Set to true if we have detected and reported DOM modification during
-   * the current paint.
-   */
-  bool                                mDetectedDOMModification;
   /**
    * Indicates that the entire layer tree should be rerendered
    * during this paint.