Paint solid color background into both scrolling contents layer and graphics 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 34be08968d9df991c67e4d51e6c74b68e868ab94..17ba956ea0b99716c94b80b12a256258f7eefb1e 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
@@ -111,7 +111,7 @@ bool LayoutBoxModelObject::usesCompositedScrolling() const {
          layer()->getScrollableArea()->usesCompositedScrolling();
 }
 
-bool LayoutBoxModelObject::hasLocalEquivalentBackground() const {
+BackgroundPaintLocation LayoutBoxModelObject::backgroundPaintLocation() const {
   int minBorderWidth = std::min(
       style()->borderTopWidth(),
       std::min(
@@ -133,14 +133,17 @@ bool LayoutBoxModelObject::hasLocalEquivalentBackground() const {
   // TODO(flackr): When we correctly clip the scrolling contents layer we can
   // paint locally equivalent backgrounds into it. https://crbug.com/645957
   if (!style()->hasAutoClip())
-    return false;
+    return BackgroundPaintInGraphicsLayer;
 
   // TODO(flackr): Remove this when box shadows are still painted correctly when
   // painting into the composited scrolling contents layer.
   // https://crbug.com/646464
   if (style()->boxShadow())
-    return false;
+    return BackgroundPaintInGraphicsLayer;
 
+  // Assume optimistically that the background can be painted in the scrolling
+  // contents until we find otherwise.
+  BackgroundPaintLocation paintLocation = BackgroundPaintInScrollingContents;
   const FillLayer* layer = &(style()->backgroundLayers());
   for (; layer; layer = layer->next()) {
     if (layer->attachment() == LocalBackgroundAttachment)
@@ -153,7 +156,7 @@ bool LayoutBoxModelObject::hasLocalEquivalentBackground() const {
     // 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;
+      return BackgroundPaintInGraphicsLayer;
 
     // Solid color layers with an effective background clip of the padding box
     // can be treated as local.
@@ -164,15 +167,25 @@ bool LayoutBoxModelObject::hasLocalEquivalentBackground() const {
         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())) {
+      if (clip == BorderFillBox) {
+        if (!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;
+        }
+        // If we have an opaque background color only, we can safely paint it
+        // into both the scrolling contents layer and the graphics layer to
+        // preserve LCD text.
+        if (layer == (&style()->backgroundLayers()) &&

[chrishtr, 2016/11/17 20:55:39]

What if there are later layers in the iteration that paint on top of this
background color?

[flackr, 2016/11/17 23:38:00]

The background color is always the last layer, there is a check that this is
indeed the last layer on line 163. So this case of painting to both layers only
happens when this is the only layer in the layer list.

+            resolveColor(CSSPropertyBackgroundColor).alpha() < 255)
+          return BackgroundPaintInGraphicsLayer;
+        paintLocation |= BackgroundPaintInGraphicsLayer;
         continue;
       }
       // A content fill box can be treated as a padding fill box if there is no
@@ -183,9 +196,9 @@ bool LayoutBoxModelObject::hasLocalEquivalentBackground() const {
         continue;
       }
     }
-    return false;
+    return BackgroundPaintInGraphicsLayer;
   }
-  return true;
+  return paintLocation;
 }
 
 LayoutBoxModelObject::~LayoutBoxModelObject() {