Refactor root element background painting

Source/core/layout/LayoutView.cpp

Index: Source/core/layout/LayoutView.cpp
diff --git a/Source/core/layout/LayoutView.cpp b/Source/core/layout/LayoutView.cpp
index f100481a8ab8727510cb2745c1dc77de5c6c09f4..9dd4ec39aeb0675d2c7a01af2b9b1ad176fd1327 100644
--- a/Source/core/layout/LayoutView.cpp
+++ b/Source/core/layout/LayoutView.cpp
@@ -195,16 +195,14 @@ bool LayoutView::shouldDoFullPaintInvalidationForNextLayout() const
         return true;
 
     if (size().height() != viewLogicalHeightForBoxSizing()) {
-        if (LayoutObject* backgroundLayoutObject = this->backgroundLayoutObject()) {
-            // When background-attachment is 'fixed', we treat the viewport (instead of the 'root'
-            // i.e. html or body) as the background positioning area, and we should full paint invalidation
-            // viewport resize if the background image is not composited and needs full paint invalidation on
-            // background positioning area resize.
-            if (!m_compositor || !m_compositor->needsFixedRootBackgroundLayer(layer())) {
-                if (backgroundLayoutObject->style()->hasFixedBackgroundImage()
-                    && mustInvalidateFillLayersPaintOnHeightChange(backgroundLayoutObject->style()->backgroundLayers()))
-                    return true;
-            }
+        // When background-attachment is 'fixed', we treat the viewport (instead of the 'root'
+        // i.e. html or body) as the background positioning area, and we should full paint invalidation
+        // viewport resize if the background image is not composited and needs full paint invalidation on
+        // background positioning area resize.
+        if (!m_compositor || !m_compositor->needsFixedRootBackgroundLayer(layer())) {
+            if (style()->hasFixedBackgroundImage()
+                && mustInvalidateFillLayersPaintOnHeightChange(style()->backgroundLayers()))
+                return true;
         }
     }
 
@@ -221,8 +219,6 @@ void LayoutView::layout()
 
     SubtreeLayoutScope layoutScope(*this);
 
-    LayoutRect oldLayoutOverflowRect = layoutOverflowRect();
-
     // Use calcWidth/Height to get the new width/height, since this will take the full page zoom factor into account.
     bool relayoutChildren = !shouldUsePrintingLayout() && (!m_frameView
         || logicalWidth() != viewLogicalWidthForBoxSizing()
@@ -254,22 +250,71 @@ void LayoutView::layout()
 
     layoutContent();
 
-    if (RuntimeEnabledFeatures::slimmingPaintEnabled() && layoutOverflowRect() != oldLayoutOverflowRect) {
-        // The document element paints the viewport background, so we need to invalidate it when
-        // layout overflow changes.
-        // FIXME: Improve viewport background styling/invalidation/painting. crbug.com/475115
-        if (Element* documentElement = document().documentElement()) {
-            if (LayoutObject* rootObject = documentElement->layoutObject())
-                rootObject->setShouldDoFullPaintInvalidation();
-        }
-    }
-
 #if ENABLE(ASSERT)
     checkLayoutState();
 #endif
     clearNeedsLayout();
 }
 
+void LayoutView::styleDidChange(StyleDifference diff, const ComputedStyle* oldStyle)
+{
+    const ComputedStyle& newStyle = styleRef();
+
+    if (oldStyle && oldStyle->hasEntirelyFixedBackground() != newStyle.hasEntirelyFixedBackground())
+        compositor()->setNeedsUpdateFixedBackground();
+
+    LayoutBlockFlow::styleDidChange(diff, oldStyle);
+}
+
+void LayoutView::getBackgroundPositioningFromDocumentElement(BackgroundPositioning& position) const
+{
+    const LayoutObject* rootObject = document().documentElement() ? document().documentElement()->layoutObject() : nullptr;
+    if (!rootObject || !rootObject->isBox())
+        return;
+
+    const LayoutBox& rootBox = toLayoutBox(*rootObject);
+    position.borderBoxSize = rootBox.size();
+    position.paddingBoxRect = rootBox.paddingBoxRect();
+    position.contentBoxRect = rootBox.contentBoxRect();
+    if (rootBox.hasLayer()) {
+        const DeprecatedPaintLayer& rootLayer = *rootBox.layer();
+        LayoutPoint offset;
+        rootLayer.convertToLayerCoords(nullptr, offset);
+        position.transform.translate(offset.x(), offset.y());
+        position.transform.multiply(rootLayer.currentTransform());
+    }
+}
+
+PaintInvalidationReason LayoutView::paintInvalidationReason(const LayoutBoxModelObject& paintInvalidationContainer,
+    const LayoutRect& oldBounds, const LayoutPoint& oldLocation,
+    const LayoutRect& newBounds, const LayoutPoint& newLocation) const
+{
+    const BackgroundPositioning& oldPosition = m_previousRootBackgroundPositioning;
+    BackgroundPositioning newPosition;
+    getBackgroundPositioningFromDocumentElement(newPosition);
+
+    if (oldPosition.transform != newPosition.transform)
+        return PaintInvalidationLocationChange;
+
+    const FillLayer& backgroundLayers = style()->backgroundLayers();
+    if (mustInvalidateFillLayersPaintOnWidthChange(backgroundLayers) || mustInvalidateFillLayersPaintOnHeightChange(backgroundLayers)) {
+        if (oldPosition.borderBoxSize != newPosition.borderBoxSize
+            || oldPosition.paddingBoxRect != newPosition.paddingBoxRect
+            || oldPosition.contentBoxRect != newPosition.contentBoxRect) {
+            return PaintInvalidationBorderBoxChange;
+        }
+    }
+
+    return LayoutBlockFlow::paintInvalidationReason(paintInvalidationContainer, oldBounds, oldLocation, newBounds, newLocation);
+}
+
+PaintInvalidationReason LayoutView::invalidatePaintIfNeeded(PaintInvalidationState& paintInvalidationState, const LayoutBoxModelObject& newPaintInvalidationContainer)
+{
+    getBackgroundPositioningFromDocumentElement(m_previousRootBackgroundPositioning);

[Xianzhu, 2015/06/05 05:08:28]

This seems incorrect. m_previousRootBackgroundPositioning and newPosition will
be got in the same cycle and always equal.

I think it would be better to do this in LayoutView::layout(). We can save old
position in stack before layoutContent() and get new position after, and check
the positions and set paint invalidation flags. 

For rootObject transform, I think we can check it in
LayoutObject::styleWillChange() (by checking
styleDifference.transformChanged()). Though we will invalidate when both offset
and transform change but the combined transform doesn't change, I think this is
acceptable.

In addition, I suggest to separate these code into another CL and add tests.
This CL has been too big.

+
+    return LayoutBlockFlow::invalidatePaintIfNeeded(paintInvalidationState, newPaintInvalidationContainer);
+}
+
 void LayoutView::mapLocalToContainer(const LayoutBoxModelObject* paintInvalidationContainer, TransformState& transformState, MapCoordinatesFlags mode, bool* wasFixed, const PaintInvalidationState* paintInvalidationState) const
 {
     ASSERT_UNUSED(wasFixed, !wasFixed || *wasFixed == static_cast<bool>(mode & IsFixed));
@@ -844,18 +889,7 @@ IntRect LayoutView::unscaledDocumentRect() const
 
 bool LayoutView::rootBackgroundIsEntirelyFixed() const
 {
-    if (LayoutObject* backgroundLayoutObject = this->backgroundLayoutObject())
-        return backgroundLayoutObject->style()->hasEntirelyFixedBackground();
-    return false;
-}
-
-LayoutObject* LayoutView::backgroundLayoutObject() const
-{
-    if (Element* documentElement = document().documentElement()) {
-        if (LayoutObject* rootObject = documentElement->layoutObject())
-            return rootObject->layoutObjectForRootBackground();
-    }
-    return 0;
+    return style()->hasEntirelyFixedBackground();
 }
 
 LayoutRect LayoutView::backgroundRect(LayoutBox* backgroundLayoutObject) const