Fixing getComputedStyle to return pixel values for left / right / top / bottom

Source/core/css/CSSComputedStyleDeclaration.cpp

Index: Source/core/css/CSSComputedStyleDeclaration.cpp
diff --git a/Source/core/css/CSSComputedStyleDeclaration.cpp b/Source/core/css/CSSComputedStyleDeclaration.cpp
index 4d09b550a6c6ebe5dc4fdd96a96c4d0531c439b0..766a2de1c47dd7d159f825ff0e7cabd9b7ffa12d 100644
--- a/Source/core/css/CSSComputedStyleDeclaration.cpp
+++ b/Source/core/css/CSSComputedStyleDeclaration.cpp
@@ -623,46 +623,107 @@ static PassRefPtr<CSSValueList> createPositionListForLayer(CSSPropertyID propert
     }
     positionList->append(zoomAdjustedPixelValueForLength(layer->yPosition(), style));
     return positionList.release();
+
 }
 
-static PassRefPtr<CSSValue> getPositionOffsetValue(RenderStyle* style, CSSPropertyID propertyID, const RenderObject* renderer, RenderView* renderView)
+static Length getOffsetComputedStyle(RenderStyle* style, CSSPropertyID propertyID)
 {
-    if (!style)
-        return 0;
+    // If specified as a length, the corresponding absolute length; if specified as
+    // a percentage, the specified value; otherwise, 'auto'. Hence, we can just
+    // return the value in the style.
+    //
+    // See http://www.w3.org/TR/CSS21/cascade.html#computed-value
+    switch (propertyID) {
+    case CSSPropertyLeft:
+        return style->left();
+    case CSSPropertyRight:
+        return style->right();
+    case CSSPropertyTop:
+        return style->top();
+    case CSSPropertyBottom:
+        return style->bottom();
+    default:
+        ASSERT_NOT_REACHED();
+    }
 
-    Length l;
+    return Length(0);
+}
+
+static LayoutUnit getOffsetUsedStyleRelative(RenderBox* box, RenderStyle* style, CSSPropertyID propertyID)
+{
+    // For relatively positioned boxes, the offset is with respect to the top edges
+    // of the box itself. This ties together top/bottom and left/right to be
+    // opposites of each other.
+    //
+    // See http://www.w3.org/TR/CSS2/visuren.html#relative-positioning
+    //
+    // Specifically;
+    //   Since boxes are not split or stretched as a result of 'left' or
+    //   'right', the used values are always: left = -right.
+    // and
+    //   Since boxes are not split or stretched as a result of 'top' or
+    //   'bottom', the used values are always: top = -bottom.
     switch (propertyID) {
-        case CSSPropertyLeft:
-            l = style->left();
-            break;
-        case CSSPropertyRight:
-            l = style->right();
-            break;
-        case CSSPropertyTop:
-            l = style->top();
-            break;
-        case CSSPropertyBottom:
-            l = style->bottom();
-            break;
-        default:
-            return 0;
+    case CSSPropertyTop:
+        return box->relativePositionOffset().height();
+    case CSSPropertyBottom:
+        return -(box->relativePositionOffset().height());
+    case CSSPropertyLeft:
+        return box->relativePositionOffset().width();
+    case CSSPropertyRight:
+        return -(box->relativePositionOffset().width());
+    default:
+        ASSERT_NOT_REACHED();
     }
 
-    if (l.isPercent() && renderer && renderer->isBox()) {
-        LayoutUnit containingBlockSize = (propertyID == CSSPropertyLeft || propertyID == CSSPropertyRight) ?
-            toRenderBox(renderer)->containingBlockLogicalWidthForContent() :
-            toRenderBox(renderer)->containingBlockLogicalHeightForContent(ExcludeMarginBorderPadding);
-        return zoomAdjustedPixelValue(valueForLength(l, containingBlockSize, 0), style);
-    } if (l.isViewportPercentage())
-        return zoomAdjustedPixelValue(valueForLength(l, 0, renderView), style);
-    if (l.isAuto()) {
-        // FIXME: It's not enough to simply return "auto" values for one offset if the other side is defined.
-        // In other words if left is auto and right is not auto, then left's computed value is negative right().
-        // So we should get the opposite length unit and see if it is auto.
-        return cssValuePool().createValue(l);
+    return 0;
+}
+
+static LayoutUnit getOffsetUsedStyleAbsolute(RenderBlock* container, RenderBox* box, RenderStyle* style, CSSPropertyID propertyID)
+{
+    // For absoultely positioned boxes, the offset is how far an box's margin
+    // edge is offset below the edge of the box's containing block.
+    // See http://www.w3.org/TR/CSS2/visuren.html#position-props
+
+    // Margins are included in Webkit's offsetTop/offsetLeft so we need to
+    // remove them here.
+    switch (propertyID) {
+    case CSSPropertyTop:
+        return box->offsetTop() - box->marginTop();
+    case CSSPropertyBottom:
+        return container->clientHeight() - (box->offsetTop() + box->offsetHeight()) - box->marginBottom();
+    case CSSPropertyLeft:
+        return box->offsetLeft() - box->marginLeft();
+    case CSSPropertyRight:
+        return container->clientWidth() - (box->offsetLeft() + box->offsetWidth()) - box->marginRight();
+    default:
+        ASSERT_NOT_REACHED();
     }
 
-    return zoomAdjustedPixelValueForLength(l, style);
+    return 0;
+}
+
+static PassRefPtr<CSSValue> getPositionOffsetValue(RenderStyle* style, CSSPropertyID propertyID, RenderObject* renderer, RenderView* renderView)
+{
+    if (!style)
+        return 0;
+
+    // If the element is not displayed; return the "computed value".
+    if (!renderView || !renderer || !renderer->isBox()) {
+        return zoomAdjustedPixelValueForLength(getOffsetComputedStyle(style, propertyID), style);
+
+    // We should return the "used value".
+    } else {
+        LayoutUnit length = 0;
+        RenderBox* box = toRenderBox(renderer);
+        RenderBlock* containingBlock = renderer->containingBlock();
+        if (box->isRelPositioned() || !containingBlock) {
+            length = getOffsetUsedStyleRelative(box, style, propertyID);
+        } else {
+            length = getOffsetUsedStyleAbsolute(containingBlock, box, style, propertyID);
+        }
+        return zoomAdjustedPixelValue(length, style);
+    }
 }
 
 PassRefPtr<CSSPrimitiveValue> CSSComputedStyleDeclaration::currentColorOrValidColor(RenderStyle* style, const Color& color) const
@@ -1508,6 +1569,13 @@ Node* CSSComputedStyleDeclaration::styledNode() const
     return m_node.get();
 }
 
+// In CSS 2.1 the returned object should actually contain the "used values"
+// rather then the "computed values" (despite the name saying otherwise).
+//
+// See;
+// http://www.w3.org/TR/CSS21/cascade.html#used-value
+// http://www.w3.org/TR/DOM-Level-2-Style/css.html#CSS-CSSStyleDeclaration
+// https://developer.mozilla.org/en-US/docs/DOM/window.getComputedStyle
 PassRefPtr<CSSValue> CSSComputedStyleDeclaration::getPropertyCSSValue(CSSPropertyID propertyID, EUpdateLayout updateLayout) const
 {
     Node* styledNode = this->styledNode();