Generate scroll/clip display item hierarchy for SPv2

Source/core/paint/DeprecatedPaintLayer.h

Index: Source/core/paint/DeprecatedPaintLayer.h
diff --git a/Source/core/paint/DeprecatedPaintLayer.h b/Source/core/paint/DeprecatedPaintLayer.h
index 07155d41d34941c5cb88ad850524513c4390f61a..de5f19551991732529811697e8cd62967cb37592 100644
--- a/Source/core/paint/DeprecatedPaintLayer.h
+++ b/Source/core/paint/DeprecatedPaintLayer.h
@@ -142,7 +142,15 @@ public:
         return curr;
     }
 
-    const LayoutPoint& location() const { ASSERT(!m_needsPositionUpdate); return m_location; }
+    enum LocationQueryBehavior {
+        IncludeScroll,
+        ExcludeScroll
+    };
+    LayoutPoint location(LocationQueryBehavior behavior = IncludeScroll) const { ASSERT(!m_needsPositionUpdate); return behavior == IncludeScroll ? m_location : locationExcludeOverflowScroll(); }

[pdr., 2015/09/03 06:16:59]

With SPV2, won't all location(...)/convertToLayerCoords(...)/etc use
ExcludeScroll? If so, can we refactor these functions like so:
LayoutPoint location() const
{
    ...
    if (!RuntimeEnabledFeatures::slimmingPaintV2Enabled())
        return m_location;

    // Compute and return locationExcludeOverflowScroll here.
}

And if we can do this, can we write unittests for these functions and land these
changes before the big patch?

[trchen, 2015/09/04 06:10:14]

Better, we can exclude scroll offset in the beginning by changing
DPL::updateLayerPosition().

There are many things that still rely on DPL geometry, e.g. hit-testing. IMO the
smoother way to proceed is to gradually migrate to SPv2, then delete the
IncludeScroll version afterwards.

+    // Note: This function is only needed during the transition period to SPv2.
+    // Once we have eliminated layer-based hit testing and clipping,
+    // we can cache the unscrolled location instead.
+    LayoutPoint locationExcludeOverflowScroll() const;
     // FIXME: size() should ASSERT(!m_needsPositionUpdate) as well, but that fails in some tests,
     // for example, fast/repaint/clipped-relative.html.
     const IntSize& size() const { return m_size; }
@@ -227,7 +235,7 @@ public:
         return !layoutObject()->hasTransformRelatedProperty() && !layoutObject()->isSVGRoot();
     }
 
-    void convertToLayerCoords(const DeprecatedPaintLayer* ancestorLayer, LayoutPoint&) const;
+    void convertToLayerCoords(const DeprecatedPaintLayer* ancestorLayer, LayoutPoint&, LocationQueryBehavior = IncludeScroll) const;
     void convertToLayerCoords(const DeprecatedPaintLayer* ancestorLayer, LayoutRect&) const;
 
     // Does the same as convertToLayerCoords() when not in multicol. For multicol, however,
@@ -245,7 +253,7 @@ public:
 
     // Bounding box relative to some ancestor layer. Pass offsetFromRoot if known.
     LayoutRect physicalBoundingBox(const DeprecatedPaintLayer* ancestorLayer, const LayoutPoint* offsetFromRoot = 0) const;
-    LayoutRect physicalBoundingBoxIncludingReflectionAndStackingChildren(const DeprecatedPaintLayer* ancestorLayer, const LayoutPoint& offsetFromRoot) const;
+    LayoutRect physicalBoundingBoxIncludingReflectionAndStackingChildren(const LayoutPoint& offsetFromRoot) const;

[pdr., 2015/09/03 06:16:59]

Lets pull the physicalBoundingBoxIncludingReflectionAndStackingChildren change
into a separate patch.

[trchen, 2015/09/04 06:10:14]

Good point! Done.

     LayoutRect fragmentsBoundingBox(const DeprecatedPaintLayer* ancestorLayer) const;
 
     LayoutRect boundingBoxForCompositingOverlapTest() const;