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..61fc55728919a5e585bc53c26a94f0b4c49ab354 100644
--- a/third_party/WebKit/Source/core/css/ComputedStyleCSSValueMapping.cpp
+++ b/third_party/WebKit/Source/core/css/ComputedStyleCSSValueMapping.cpp
@@ -222,6 +222,11 @@ static CSSValue* ValueForPositionOffset(const ComputedStyle& style,
   }
 
   if (offset.IsAuto() && layout_object) {
+    // "top:auto" and "top:0px" have very different meaning for sticky elements.
+    // Therefore "auto" should be returned regardless the opposite.

[flackr, 2017/05/09 20:26:34]

nit: Add a link to the bug for reference.

[yigu, 2017/05/09 20:43:58]

Done.

+    if (layout_object->IsStickyPositioned())
+      return CSSIdentifierValue::Create(CSSValueAuto);
+
     // 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()) {

[flackr, 2017/05/09 20:26:35]

Sticky is in flow positioned, let's change this condition to not apply to sticky
position (i.e. relative position only) and let sticky return auto from line 299
below.

[yigu, 2017/05/09 20:43:58]

Done.