Paint solid color backgrounds which are equivalent to locally attached into scrolling contents layer

third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp

Index: third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp b/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
index 6c6ec933ce0598843aae45578a2128c3483ef273..a757f5e362f632e2d3b2cd70e1061308f658b884 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
@@ -33,6 +33,7 @@
 #include "core/layout/LayoutFlexibleBox.h"
 #include "core/layout/LayoutGeometryMap.h"
 #include "core/layout/LayoutInline.h"
+#include "core/layout/LayoutTheme.h"
 #include "core/layout/LayoutView.h"
 #include "core/layout/compositing/CompositedLayerMapping.h"
 #include "core/layout/compositing/PaintLayerCompositor.h"
@@ -115,6 +116,64 @@ bool LayoutBoxModelObject::usesCompositedScrolling() const
     return hasOverflowClip() && hasLayer() && layer()->getScrollableArea()->usesCompositedScrolling();
 }
 
+bool LayoutBoxModelObject::hasLocalEquivalentBackground() const
+{
+    int minBorderWidth = std::min(style()->borderTopWidth(),
+        std::min(style()->borderLeftWidth(),
+            std::min(style()->borderRightWidth(), style()->borderBottomWidth())));
+    bool outlineOverlapsPaddingBox = style()->outlineOffset() < -minBorderWidth;
+    bool hasCustomScrollbars = false;
+    // TODO(flackr): Detect opaque custom scrollbars which would cover up a border-box
+    // background.
+    if (PaintLayerScrollableArea* scrollableArea = getScrollableArea()) {
+        if ((scrollableArea->horizontalScrollbar() && scrollableArea->horizontalScrollbar()->isCustomScrollbar())
+            || (scrollableArea->verticalScrollbar() && scrollableArea->verticalScrollbar()->isCustomScrollbar())) {
+            hasCustomScrollbars = true;
+        }
+    }
+
+    const FillLayer* layer = &(style()->backgroundLayers());
+    for (; layer; layer = layer->next()) {
+        if (layer->attachment() == LocalBackgroundAttachment)
+            continue;
+
+        // If the outline draws inside the border, it intends to draw on top of the scroller's background,
+        // however because it is painted into a layer behind the scrolling contents layer we avoid auto
+        // promoting in this case to avoid obscuring the outline.
+        // TODO(flackr): Outlines should be drawn on top of the scrolling contents layer so that
+        // they cannot be covered up by composited scrolling contents.
+        if (outlineOverlapsPaddingBox)
+            return false;
+
+        // Solid color layers with an effective background clip of the padding box can be treated
+        // as local.
+        if (!layer->image() && !layer->next() && resolveColor(CSSPropertyBackgroundColor).alpha() > 0) {
+            EFillBox clip = layer->clip();
+            if (clip == PaddingFillBox)
+                continue;
+            // A border box can be treated as a padding box if the border is opaque or there is
+            // no border and we don't have custom scrollbars.
+            if (clip == BorderFillBox && !hasCustomScrollbars
+                && (style()->borderTopWidth() == 0 || !resolveColor(CSSPropertyBorderTopColor).hasAlpha())
+                && (style()->borderLeftWidth() == 0 || !resolveColor(CSSPropertyBorderLeftColor).hasAlpha())
+                && (style()->borderRightWidth() == 0 || !resolveColor(CSSPropertyBorderRightColor).hasAlpha())
+                && (style()->borderBottomWidth() == 0 || !resolveColor(CSSPropertyBorderBottomColor).hasAlpha())) {
+                continue;
+            }
+            // A content fill box can be treated as a padding fill box if there is no padding.
+            if (clip == ContentFillBox
+                && style()->paddingTop().isZero()
+                && style()->paddingLeft().isZero()
+                && style()->paddingRight().isZero()
+                && style()->paddingBottom().isZero()) {
+                continue;
+            }
+        }
+        return false;
+    }
+    return true;
+}
+
 LayoutBoxModelObject::~LayoutBoxModelObject()
 {
     // Our layer should have been destroyed and cleared by now