Only update composited scrolling state when necessary

Source/WebCore/rendering/RenderView.cpp

Index: Source/WebCore/rendering/RenderView.cpp
diff --git a/Source/WebCore/rendering/RenderView.cpp b/Source/WebCore/rendering/RenderView.cpp
index 95c9717d1b124c324aab9ce1bdc46fb7d233ac2b..2cfa55531b7be9fe9d42a5c3e719c1a822f138c7 100644
--- a/Source/WebCore/rendering/RenderView.cpp
+++ b/Source/WebCore/rendering/RenderView.cpp
@@ -63,6 +63,7 @@ RenderView::RenderView(Document* document)
     , m_pageLogicalHeightChanged(false)
     , m_layoutState(0)
     , m_layoutStateDisableCount(0)
+    , m_scopedCompositedScrollingUpdatersCount(0)
     , m_renderQuoteHead(0)
     , m_renderCounterCount(0)
 {
@@ -147,6 +148,12 @@ static RenderBox* enclosingSeamlessRenderer(Document* doc)
     return ownerElement->renderBox();
 }
 
+void RenderView::cancelScheduledCompositedScrollingUpdatesForLayer(RenderLayer* layer)
+{
+    m_pendingContiguityUpdates.remove(layer);
+    m_pendingCompositedScrollingUpdates.remove(layer);
+}
+
 void RenderView::addChild(RenderObject* newChild, RenderObject* beforeChild)
 {
     // Seamless iframes are considered part of an enclosing render flow thread from the parent document. This is necessary for them to look
@@ -1172,4 +1179,59 @@ FragmentationDisabler::~FragmentationDisabler()
         m_root->setFlowThreadStateIncludingDescendants(m_flowThreadState);
 }
 
+ScopedCompositedScrollingUpdater::ScopedCompositedScrollingUpdater(RenderView* renderView)
+    : m_renderView(renderView)
+{
+    if (m_renderView)

[Julien - ping for review, 2013/04/09 21:06:43]

Cannot be NULL per your explanation.

+        m_renderView->m_scopedCompositedScrollingUpdatersCount++;
+}
+
+ScopedCompositedScrollingUpdater::ScopedCompositedScrollingUpdater(RenderLayer* layer, CompositedScrollingUpdateMode mode)
+    : m_renderView(0)
+{
+    if (!layer || !layer->acceleratedCompositingForOverflowScrollEnabled())
+        return;
+
+    m_renderView = layer->renderer()->view();
+    m_renderView->m_scopedCompositedScrollingUpdatersCount++;
+
+    if (mode == UpdateNeedsCompositedScrolling)
+        m_renderView->m_pendingCompositedScrollingUpdates.add(layer);
+    else if (mode == UpdateDescendantsAreContiguousInStackingOrder)
+        m_renderView->m_pendingContiguityUpdates.add(layer);
+}
+
+ScopedCompositedScrollingUpdater::~ScopedCompositedScrollingUpdater()
+{
+    if (!m_renderView)

[Julien - ping for review, 2013/04/09 21:06:43]

Cannot be NULL per your explanation.

+        return;
+
+    m_renderView->m_scopedCompositedScrollingUpdatersCount--;
+    ASSERT(m_renderView->m_scopedCompositedScrollingUpdatersCount >= 0);
+    if (!m_renderView->m_scopedCompositedScrollingUpdatersCount) {

[Julien - ping for review, 2013/04/09 21:06:43]

Early return.

+        HashSet<RenderLayer*> pendingCompositedScrollingUpdates = m_renderView->m_pendingCompositedScrollingUpdates;
+        m_renderView->m_pendingCompositedScrollingUpdates.clear();
+
+        HashSet<RenderLayer*> pendingContiguityUpdates = m_renderView->m_pendingContiguityUpdates;
+        m_renderView->m_pendingContiguityUpdates.clear();
+
+        if (!pendingContiguityUpdates.isEmpty()) {

[Julien - ping for review, 2013/04/09 21:06:43]

I don't think we need an isEmpty check as the iterator would just bail out
below.

+            HashSet<RenderLayer*> stackingContexts;
+            for (HashSet<RenderLayer*>::iterator iter = pendingContiguityUpdates.begin(); iter != pendingContiguityUpdates.end(); ++iter) {
+                if (RenderLayer* stackingContext = (*iter)->enclosingStackingContext())
+                    stackingContexts.add(stackingContext);
+            }
+            for (HashSet<RenderLayer*>::iterator iter = stackingContexts.begin(); iter != stackingContexts.end(); ++iter) {
+                (*iter)->updateLayerListsIfNeeded();
+                (*iter)->updateDescendantsAreContiguousInStackingOrder();
+            }
+        }
+
+        if (!pendingCompositedScrollingUpdates.isEmpty()) {

[Julien - ping for review, 2013/04/09 21:06:43]

Same comment.

+            for (HashSet<RenderLayer*>::iterator iter = pendingCompositedScrollingUpdates.begin(); iter != pendingCompositedScrollingUpdates.end(); ++iter)
+                (*iter)->updateNeedsCompositedScrolling();
+        }
+    }

[Julien - ping for review, 2013/04/09 21:06:43]

You do store the HashSet into local variables and clear them. It's good
defense-in-depth coding but we would probably ASSERT here that no one updated
our lists or else we would miss their update indefinitely.

+}
+
 } // namespace WebCore