Paint local attachment backgrounds into composited scrolling contents layer

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

Index: third_party/WebKit/Source/core/layout/LayoutBoxTest.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutBoxTest.cpp b/third_party/WebKit/Source/core/layout/LayoutBoxTest.cpp
index e89af5b107787ae323f747ebc74f500f58fcfcd8..b9e0594fea9ce6443fe887d85c683c5804e638e8 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBoxTest.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBoxTest.cpp
@@ -24,4 +24,47 @@ TEST_F(LayoutBoxTest, BackgroundObscuredInRect)
     ASSERT_TRUE(layoutObject->backgroundIsKnownToBeObscured());
 }
 
+TEST_F(LayoutBoxTest, BackgroundRect)
+{
+    setBodyInnerHTML("<style>div { position: absolute; width: 100px; height: 100px; padding: 10px; border: 10px solid black; overflow: scroll; }"
+        "#target1 { background: url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUg) border-box, green content-box;}"
+        "#target2 { background: url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUg) content-box, green local border-box;}"
+        "#target3 { background: url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUg) content-box, rgba(0, 255, 0, 0.5) border-box;}"
+        "#target4 { background: url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUg) content-box, green border-box multiply;}"

[trchen, 2016/08/17 01:22:55]

The shorthand form doesn't seem to support background-blend-mode. You may have
to write "background-blend-mode:normal,multiply;".

+        "#target5 { background: none border-box, green content-box;}"
+        "</style>"
+        "<div id='target1'></div>"
+        "<div id='target2'></div>"
+        "<div id='target3'></div>"
+        "<div id='target4'></div>"
+        "<div id='target5'></div>");
+
+    // #target1's opaque background color only fills the content box but its translucent image extends to the borders.
+    LayoutBox* layoutBox = toLayoutBox(getLayoutObjectByElementId("target1"));
+    EXPECT_EQ(LayoutRect(20, 20, 100, 100), layoutBox->backgroundRect(BackgroundKnownOpaqueRect));
+    EXPECT_EQ(LayoutRect(0, 0, 140, 140), layoutBox->backgroundRect(BackgroundClipRect));
+
+    // #target2's background color is opaque but only fills the padding-box because it has local attachment.
+    // This eclipses the content-box image.
+    layoutBox = toLayoutBox(getLayoutObjectByElementId("target2"));
+    EXPECT_EQ(LayoutRect(10, 10, 120, 120), layoutBox->backgroundRect(BackgroundKnownOpaqueRect));
+    EXPECT_EQ(LayoutRect(10, 10, 120, 120), layoutBox->backgroundRect(BackgroundClipRect));
+
+    // #target3's background color is not opaque so we only have a clip rect.
+    layoutBox = toLayoutBox(getLayoutObjectByElementId("target3"));
+    EXPECT_TRUE(layoutBox->backgroundRect(BackgroundKnownOpaqueRect).isEmpty());
+    EXPECT_EQ(LayoutRect(0, 0, 140, 140), layoutBox->backgroundRect(BackgroundClipRect));
+
+    // #target4's background color has a blend mode so it isn't opaque.

[trchen, 2016/08/17 01:22:55]

Note: CSS backgrounds always create an isolated group, so blending should not
affect background opaqueness (as a whole) in any way. It is okay to be
conservative though.

+    layoutBox = toLayoutBox(getLayoutObjectByElementId("target4"));
+    EXPECT_TRUE(layoutBox->backgroundRect(BackgroundKnownOpaqueRect).isEmpty());
+    EXPECT_EQ(LayoutRect(0, 0, 140, 140), layoutBox->backgroundRect(BackgroundClipRect));
+
+    // #target5's solid background only covers the content-box but it has a "none" background
+    // covering the border box.
+    layoutBox = toLayoutBox(getLayoutObjectByElementId("target5"));
+    EXPECT_EQ(LayoutRect(20, 20, 100, 100), layoutBox->backgroundRect(BackgroundKnownOpaqueRect));
+    EXPECT_EQ(LayoutRect(0, 0, 140, 140), layoutBox->backgroundRect(BackgroundClipRect));

[trchen, 2016/08/17 01:22:55]

This is surprising to me. I think it relies on the fact that
FillLayer::cullEmptyLayers() cannot delete the top layer. If you insert one
layer in the beginning like: "background: none content-box, none border-box,
green content-box;", the result probably will be different.

It is not your fault, but a comment here can save a lot of head scratching. :)

+}
+
 } // namespace blink