Root layer scrolling: set correct size for document's main GraphicsLayer.

third_party/WebKit/Source/core/paint/ViewPainter.cpp

Index: third_party/WebKit/Source/core/paint/ViewPainter.cpp
diff --git a/third_party/WebKit/Source/core/paint/ViewPainter.cpp b/third_party/WebKit/Source/core/paint/ViewPainter.cpp
index a3fa2e373dcb74a684f18143ffbb4954ba3a4590..3798a1fa098fc22ec9b0a7b9627ad1e92aea22e0 100644
--- a/third_party/WebKit/Source/core/paint/ViewPainter.cpp
+++ b/third_party/WebKit/Source/core/paint/ViewPainter.cpp
@@ -56,7 +56,11 @@ void ViewPainter::paintBoxDecorationBackground(const PaintInfo& paintInfo)
     if (LayoutObjectDrawingRecorder::useCachedDrawingIfPossible(context, m_layoutView, DisplayItem::BoxDecorationBackground, LayoutPoint()))
         return;
 
-    IntRect documentRect = m_layoutView.documentRect();
+    // When the view uses composited scrolling, the main GraphicsLayer (where the background is painted)
+    // is sized to the layout viewport. Otherwise, it's the size of the document.
+    IntRect backgroundRect = m_layoutView.layer()->needsCompositedScrolling() ?
+        pixelSnappedIntRect(m_layoutView.viewRect()) : m_layoutView.documentRect();
+
     const Document& document = m_layoutView.document();
     const FrameView& frameView = *m_layoutView.frameView();
     bool isMainFrame = !document.ownerElement();
@@ -66,14 +70,14 @@ void ViewPainter::paintBoxDecorationBackground(const PaintInfo& paintInfo)
     Color rootBackgroundColor = m_layoutView.style()->visitedDependentColor(CSSPropertyBackgroundColor);
     const LayoutObject* rootObject = document.documentElement() ? document.documentElement()->layoutObject() : nullptr;
 
-    LayoutObjectDrawingRecorder recorder(context, m_layoutView, DisplayItem::BoxDecorationBackground, documentRect, LayoutPoint());
+    LayoutObjectDrawingRecorder recorder(context, m_layoutView, DisplayItem::BoxDecorationBackground, backgroundRect, LayoutPoint());
 
     // Special handling for print economy mode.
     bool forceBackgroundToWhite = BoxPainter::shouldForceWhiteBackgroundForPrintEconomy(m_layoutView.styleRef(), document);
     if (forceBackgroundToWhite) {
         // If for any reason the view background is not transparent, paint white instead, otherwise keep transparent as is.
         if (paintsBaseBackground || rootBackgroundColor.alpha() || m_layoutView.style()->backgroundLayers().image())
-            context.fillRect(documentRect, Color::white, SkXfermode::kSrc_Mode);
+            context.fillRect(backgroundRect, Color::white, SkXfermode::kSrc_Mode);
         return;
     }
 
@@ -85,7 +89,7 @@ void ViewPainter::paintBoxDecorationBackground(const PaintInfo& paintInfo)
     // we need to apply inverse transform on the document rect to get to the root element space.
     bool backgroundRenderable = true;
     TransformationMatrix transform;
-    IntRect paintRect = documentRect;
+    IntRect paintRect = backgroundRect;
     if (!rootObject || !rootObject->isBox()) {
         backgroundRenderable = false;
     } else if (rootObject->hasLayer()) {
@@ -99,16 +103,16 @@ void ViewPainter::paintBoxDecorationBackground(const PaintInfo& paintInfo)
             backgroundRenderable = false;
         } else {
             bool isClamped;
-            paintRect = transform.inverse().projectQuad(FloatQuad(documentRect), &isClamped).enclosingBoundingBox();
+            paintRect = transform.inverse().projectQuad(FloatQuad(backgroundRect), &isClamped).enclosingBoundingBox();
             backgroundRenderable = !isClamped;
         }
     }
 
     if (!backgroundRenderable) {
         if (baseBackgroundColor.alpha())
-            context.fillRect(documentRect, baseBackgroundColor, shouldClearCanvas ? SkXfermode::kSrc_Mode : SkXfermode::kSrcOver_Mode);
+            context.fillRect(backgroundRect, baseBackgroundColor, shouldClearCanvas ? SkXfermode::kSrc_Mode : SkXfermode::kSrcOver_Mode);
         else if (shouldClearCanvas)
-            context.fillRect(documentRect, Color(), SkXfermode::kClear_Mode);
+            context.fillRect(backgroundRect, Color(), SkXfermode::kClear_Mode);
         return;
     }
 
@@ -127,15 +131,15 @@ void ViewPainter::paintBoxDecorationBackground(const PaintInfo& paintInfo)
 
     if (shouldDrawBackgroundInSeparateBuffer) {
         if (baseBackgroundColor.alpha())
-            context.fillRect(documentRect, baseBackgroundColor, shouldClearCanvas ? SkXfermode::kSrc_Mode : SkXfermode::kSrcOver_Mode);
+            context.fillRect(backgroundRect, baseBackgroundColor, shouldClearCanvas ? SkXfermode::kSrc_Mode : SkXfermode::kSrcOver_Mode);
         context.beginLayer();
     }
 
     Color combinedBackgroundColor = shouldDrawBackgroundInSeparateBuffer ? rootBackgroundColor : baseBackgroundColor.blend(rootBackgroundColor);
     if (combinedBackgroundColor.alpha())
-        context.fillRect(documentRect, combinedBackgroundColor, (shouldDrawBackgroundInSeparateBuffer || shouldClearCanvas) ? SkXfermode::kSrc_Mode : SkXfermode::kSrcOver_Mode);
+        context.fillRect(backgroundRect, combinedBackgroundColor, (shouldDrawBackgroundInSeparateBuffer || shouldClearCanvas) ? SkXfermode::kSrc_Mode : SkXfermode::kSrcOver_Mode);
     else if (shouldClearCanvas && !shouldDrawBackgroundInSeparateBuffer)
-        context.fillRect(documentRect, Color(), SkXfermode::kClear_Mode);
+        context.fillRect(backgroundRect, Color(), SkXfermode::kClear_Mode);
 
     for (auto it = reversedPaintList.rbegin(); it != reversedPaintList.rend(); ++it) {
         ASSERT((*it)->clip() == BorderFillBox);