Blend background with existing content

Source/core/rendering/RenderBoxModelObject.cpp

Index: Source/core/rendering/RenderBoxModelObject.cpp
diff --git a/Source/core/rendering/RenderBoxModelObject.cpp b/Source/core/rendering/RenderBoxModelObject.cpp
index 932db84e2a01905a1926066b8f1bb04e49f1d64a..7ca5136bc25d6956be28091f906e814784e8e318 100644
--- a/Source/core/rendering/RenderBoxModelObject.cpp
+++ b/Source/core/rendering/RenderBoxModelObject.cpp
@@ -706,29 +706,28 @@ void RenderBoxModelObject::paintFillLayerExtended(const PaintInfo& paintInfo, co
             if (!boxShadowShouldBeAppliedToBackground)
                 backgroundRect.intersect(paintInfo.rect);
 
-            // If we have an alpha and we are painting the root element, go ahead and blend with the base background color.
-            Color baseColor;
-            bool shouldClearBackground = false;
-            if (isOpaqueRoot) {
-                baseColor = view()->frameView()->baseBackgroundColor();
-                if (!baseColor.alpha())
-                    shouldClearBackground = true;
-            }
-
             GraphicsContextStateSaver shadowStateSaver(*context, boxShadowShouldBeAppliedToBackground);
             if (boxShadowShouldBeAppliedToBackground)
                 applyBoxShadowForBackground(context, this);
 
-            if (baseColor.alpha()) {
-                if (bgColor.alpha())
-                    baseColor = baseColor.blend(bgColor);
-
-                context->fillRect(backgroundRect, baseColor, CompositeCopy);
+            if (isOpaqueRoot) {
+                // If we have an alpha and we are painting the root element, go ahead and blend with the base background color.
+                Color baseColor = view()->frameView()->baseBackgroundColor();
+                bool shouldBlendWithExistingContent = document().settings() && document().settings()->shouldBlendWithExistingContent();

[enne (OOO), 2013/09/24 18:35:48]

bikeshed: shouldBlendWithExistingContent is a little bit ambiguous in the
context of a document setting.  (What's being blended? What content?) Maybe
something like shouldClearDocumentBackground? Or, maybe there's some other
better name?

+                CompositeOperator operation = shouldBlendWithExistingContent ? context->compositeOperation() : CompositeCopy;
+
+                if (baseColor.alpha()) {
+                    if (bgColor.alpha())
+                        baseColor = baseColor.blend(bgColor);
+                    context->fillRect(backgroundRect, baseColor, operation);
+                } else if (bgColor.alpha()) {
+                    context->fillRect(backgroundRect, bgColor, operation);
+                } else if (!shouldBlendWithExistingContent) {
+                    context->clearRect(backgroundRect);
+                }
             } else if (bgColor.alpha()) {
-                CompositeOperator operation = shouldClearBackground ? CompositeCopy : context->compositeOperation();
-                context->fillRect(backgroundRect, bgColor, operation);
-            } else if (shouldClearBackground)
-                context->clearRect(backgroundRect);
+                context->fillRect(backgroundRect, bgColor, context->compositeOperation());
+            }
         }
     }