Update return value of getComputedStyle for sticky elements

third_party/WebKit/Source/core/css/ComputedStyleCSSValueMapping.cpp

Index: third_party/WebKit/Source/core/css/ComputedStyleCSSValueMapping.cpp
diff --git a/third_party/WebKit/Source/core/css/ComputedStyleCSSValueMapping.cpp b/third_party/WebKit/Source/core/css/ComputedStyleCSSValueMapping.cpp
index 9d4e5c14c0e8de4d66f367f8eff7ae977eaba06c..4b5ca83d0e769787379146aa4643390e7c3c0af4 100644
--- a/third_party/WebKit/Source/core/css/ComputedStyleCSSValueMapping.cpp
+++ b/third_party/WebKit/Source/core/css/ComputedStyleCSSValueMapping.cpp
@@ -224,7 +224,12 @@ static CSSValue* ValueForPositionOffset(const ComputedStyle& style,
   if (offset.IsAuto() && layout_object) {
     // If the property applies to a positioned element and the resolved value of
     // the display property is not none, the resolved value is the used value.
-    if (layout_object->IsInFlowPositioned()) {
+    // According to https://crbug.com/703816, "top:auto" and "top:0px" have very

[flackr, 2017/05/09 21:21:41]

nit: Let's just say something to the effect of "Position offsets have special
meaning for position sticky so we return auto when offset.isAuto() on a sticky
position object: https://crbug.com/703816"

[yigu, 2017/05/09 21:35:20]

Done.

+    // different meaning for sticky elements. Therefore "auto" should be
+    // returned regardless the opposite. Hence we only check IsRelPositioned
+    // instead of IsInflowPositioned so StickyPositioned element can be handled
+    // implicitly.
+    if (layout_object->IsRelPositioned()) {
       // If e.g. left is auto and right is not auto, then left's computed value
       // is negative right. So we get the opposite length unit and see if it is
       // auto.