Update return value of getComputedStyle for sticky elements

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

 /*
  * Copyright (C) 2004 Zack Rusin <zack@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc.
  * All rights reserved.
  * Copyright (C) 2007 Alexey Proskuryakov <ap@webkit.org>
  * Copyright (C) 2007 Nicholas Shanks <webkit@nickshanks.com>
  * Copyright (C) 2011 Sencha, Inc. All rights reserved.
  * Copyright (C) 2015 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 204 matching lines @@
         (property_id == CSSPropertyLeft || property_id == CSSPropertyRight)
             ? ToLayoutBox(layout_object)
                   ->ContainingBlockLogicalWidthForContent()
             : ToLayoutBox(layout_object)
                   ->ContainingBlockLogicalHeightForGetComputedStyle();
     return ZoomAdjustedPixelValue(ValueForLength(offset, containing_block_size),
                                   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.

       // 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.
       if (opposite.IsAuto())
         return CSSPrimitiveValue::Create(0,
                                          CSSPrimitiveValue::UnitType::kPixels);
 
       if (opposite.IsPercentOrCalc()) {
         if (layout_object->IsBox()) {
           LayoutUnit containing_block_size =
@@ skipping 3507 matching lines @@
     case CSSPropertyAll:
       return nullptr;
     default:
       break;
   }
   NOTREACHED();
   return nullptr;
 }
 
 }  // namespace blink