Refactor root element background painting

Source/core/dom/Document.cpp

Index: Source/core/dom/Document.cpp
diff --git a/Source/core/dom/Document.cpp b/Source/core/dom/Document.cpp
index 605ea63164fcc2ab7bfcf6708ce12dada440908a..ef81ab9a617b8402cfc9ac56d8e6b1f4049a4cf9 100644
--- a/Source/core/dom/Document.cpp
+++ b/Source/core/dom/Document.cpp
@@ -1627,16 +1627,28 @@ void Document::inheritHtmlAndBodyElementStyles(StyleRecalcChange change)
         rootDirection = bodyStyle->direction();
     }
 
-    RefPtr<ComputedStyle> overflowStyle;
+    const ComputedStyle* overflowStyle = nullptr;
     if (Element* element = viewportDefiningElement(documentElementStyle.get())) {
         if (element == body) {
-            overflowStyle = bodyStyle;
+            overflowStyle = bodyStyle.get();
         } else {
             ASSERT(element == documentElement());
-            overflowStyle = documentElementStyle;
+            overflowStyle = documentElementStyle.get();
         }
     }
 
+    const ComputedStyle* backgroundStyle = documentElementStyle.get();
+    if (isHTMLHtmlElement(documentElement()) && isHTMLBodyElement(body) && !backgroundStyle->hasBackground())
+        backgroundStyle = bodyStyle.get();
+    Color backgroundColor = backgroundStyle->visitedDependentColor(CSSPropertyBackgroundColor);
+    FillLayer backgroundLayers = backgroundStyle->backgroundLayers();
+    for (auto currentLayer = &backgroundLayers; currentLayer; currentLayer = currentLayer->next()) {
+        // http://www.w3.org/TR/css3-background/#root-background
+        // The root element background always have painting area of the whole canvas.
+        currentLayer->setClip(BorderFillBox);
+    }
+    EImageRendering imageRendering = backgroundStyle->imageRendering();
+
     // Resolved rem units are stored in the matched properties cache so we need to make sure to
     // invalidate the cache if the documentElement needed to reattach or the font size changed
     // and then trigger a full document recalc. We also need to clear it here since the
@@ -1670,9 +1682,12 @@ void Document::inheritHtmlAndBodyElementStyles(StyleRecalcChange change)
     RefPtr<ComputedStyle> documentStyle = layoutView()->mutableStyle();
     if (documentStyle->writingMode() != rootWritingMode
         || documentStyle->direction() != rootDirection
+        || documentStyle->columnGap() != columnGap
         || documentStyle->overflowX() != overflowX
         || documentStyle->overflowY() != overflowY
-        || documentStyle->columnGap() != columnGap
+        || documentStyle->visitedDependentColor(CSSPropertyBackgroundColor) != backgroundColor
+        || documentStyle->backgroundLayers() != backgroundLayers
+        || documentStyle->imageRendering() != imageRendering
         || documentStyle->scrollBlocksOn() != scrollBlocksOn) {
         RefPtr<ComputedStyle> newStyle = ComputedStyle::clone(*documentStyle);
         newStyle->setWritingMode(rootWritingMode);
@@ -1680,6 +1695,9 @@ void Document::inheritHtmlAndBodyElementStyles(StyleRecalcChange change)
         newStyle->setColumnGap(columnGap);
         newStyle->setOverflowX(overflowX);
         newStyle->setOverflowY(overflowY);
+        newStyle->setBackgroundColor(backgroundColor);
+        newStyle->accessBackgroundLayers() = backgroundLayers;
+        newStyle->setImageRendering(imageRendering);
         newStyle->setScrollBlocksOn(scrollBlocksOn);
         layoutView()->setStyle(newStyle);
         setupFontBuilder(*newStyle);