Force subtree paint property updates on local border box changes

third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilderTest.cpp

Index: third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilderTest.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilderTest.cpp b/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilderTest.cpp
index c5b5f979fe69e30528d669d964492e8869bcf052..1f3b1218edf375d5274e101e81699f6fa9d47d8c 100644
--- a/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilderTest.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilderTest.cpp
@@ -3231,4 +3231,34 @@ TEST_P(PaintPropertyTreeBuilderTest, NoPaintPropertyUpdateOnBackgroundChange) {
   EXPECT_FALSE(div->layoutObject()->needsPaintPropertyUpdate());
 }
 
+TEST_P(PaintPropertyTreeBuilderTest, LocalBorderBoxChanges) {

[chrishtr, 2016/12/15 19:42:52]

For completeness, please also add a test for position: fixed.

+  setBodyInnerHTML(
+      "<style>"
+      "  body { margin: 0 }"
+      "  #scroller { width: 3px; height: 5px; overflow: scroll; }"
+      "  #child { top: 0; left: 0; }"
+      "</style>"
+      "<div id='scroller'>"
+      "  <div id='child'>"
+      "    <div id='grandChild'></div>"
+      "  </div>"
+      "</div>");
+  document().view()->updateAllLifecyclePhases();
+  auto* grandChild = document().getElementById("grandChild");
+  auto* grandChildProperties = grandChild->layoutObject()->paintProperties();
+  auto* localBorderBox = grandChildProperties->localBorderBoxProperties();
+  EXPECT_EQ(FloatRect(0, 0, 3, 5),
+            localBorderBox->propertyTreeState.clip()->clipRect().rect());
+
+  // Update the scroller's transform value but ensure the child's paint offset
+  // is unchanged so subtree updates are not forced by paint offset changes.
+  auto* scroller = document().getElementById("scroller");
+  scroller->setScrollTop(7);
+  auto* child = document().getElementById("child");
+  child->setAttribute(HTMLNames::styleAttr, "position: absolute");
+  document().view()->updateAllLifecyclePhases();
+  EXPECT_EQ(FloatRect(0, 0, 800, 600),

[chrishtr, 2016/12/15 19:42:52]

Also check its transform node and scroll node to see that it's the root and not
the scroller.

And an example with effects?

+            localBorderBox->propertyTreeState.clip()->clipRect().rect());
+}
+
 }  // namespace blink