Have enclosingPositionedAncestor() detect whether it walked past a certain ancestor.

Source/core/paint/DeprecatedPaintLayer.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 755 matching lines @@
             }
             curr = curr->parent();
         }
         if (curr->isBox() && curr->isTableRow()) {
             // Put ourselves into the row coordinate space.
             localPoint.moveBy(-toLayoutBox(curr)->topLeftLocation());
         }
     }
 
     // Subtract our parent's scroll offset.
-    if (layoutObject()->isOutOfFlowPositioned() && enclosingPositionedAncestor()) {
-        DeprecatedPaintLayer* positionedParent = enclosingPositionedAncestor();
-
+    if (DeprecatedPaintLayer* positionedParent = layoutObject()->isOutOfFlowPositioned() ? enclosingPositionedAncestor() : nullptr) {
         // For positioned layers, we subtract out the enclosing positioned layer's scroll offset.
         if (positionedParent->layoutObject()->hasOverflowClip()) {
             IntSize offset = positionedParent->layoutBox()->scrolledContentOffset();
             localPoint -= offset;
         }
 
         if (positionedParent->layoutObject()->isRelPositioned() && positionedParent->layoutObject()->isLayoutInline()) {
             LayoutSize offset = toLayoutInline(positionedParent->layoutObject())->offsetForInFlowPositionedInline(*toLayoutBox(layoutObject()));
             localPoint += offset;
         }
@@ skipping 48 matching lines @@
     const ComputedStyle& style = layoutObject()->styleRef();
 
     return FloatPoint(floatValueForLength(style.perspectiveOriginX(), borderBox.width().toFloat()), floatValueForLength(style.perspectiveOriginY(), borderBox.height().toFloat()));
 }
 
 static inline bool isFixedPositionedContainer(DeprecatedPaintLayer* layer)
 {
     return layer->isRootLayer() || layer->hasTransformRelatedProperty();
 }
 
-DeprecatedPaintLayer* DeprecatedPaintLayer::enclosingPositionedAncestor() const
+DeprecatedPaintLayer* DeprecatedPaintLayer::enclosingPositionedAncestor(const DeprecatedPaintLayer* ancestor, bool* skippedAncestor) const
 {
+    ASSERT(!ancestor || skippedAncestor); // If we have specified an ancestor, surely the caller needs to know whether we skipped it.
+    if (skippedAncestor)
+        *skippedAncestor = false;
     if (layoutObject()->style()->position() == FixedPosition) {
         DeprecatedPaintLayer* curr = parent();
-        while (curr && !isFixedPositionedContainer(curr))
+        while (curr && !isFixedPositionedContainer(curr)) {
+            if (skippedAncestor && curr == ancestor)
+                *skippedAncestor = true;
             curr = curr->parent();
+        }
 
         return curr;
     }
 
     DeprecatedPaintLayer* curr = parent();
-    while (curr && !curr->isPositionedContainer())
+    while (curr && !curr->isPositionedContainer()) {
+        if (skippedAncestor && curr == ancestor)
+            *skippedAncestor = true;
         curr = curr->parent();
+    }
 
     return curr;
 }
 
 DeprecatedPaintLayer* DeprecatedPaintLayer::enclosingTransformedAncestor() const
 {
     DeprecatedPaintLayer* curr = parent();
     while (curr && !curr->isRootLayer() && !curr->layoutObject()->hasTransformRelatedProperty())
         curr = curr->parent();
 
@@ skipping 443 matching lines @@
             ASSERT(ancestorLayer != layoutObject->view()->layer());
             ASSERT(ancestorLayer->hasTransformRelatedProperty());
 
             location += layer->location();
         }
         return foundAncestor ? ancestorLayer : fixedPositionContainerLayer;
     }
 
     DeprecatedPaintLayer* parentLayer;
     if (position == AbsolutePosition) {
-        // Do what enclosingPositionedAncestor() does, but check for ancestorLayer along the way.
-        parentLayer = layer->parent();
-        bool foundAncestorFirst = false;
-        while (parentLayer) {
-            if (parentLayer->isPositionedContainer())
-                break;
-
-            if (parentLayer == ancestorLayer) {
-                foundAncestorFirst = true;
-                break;
-            }
-
-            parentLayer = parentLayer->parent();
-        }
+        bool foundAncestorFirst;
+        parentLayer = layer->enclosingPositionedAncestor(ancestorLayer, &foundAncestorFirst);
 
         if (foundAncestorFirst) {
             // Found ancestorLayer before the abs. positioned container, so compute offset of both relative
-            // to enclosingPositionedAncestor and subtract.
-            DeprecatedPaintLayer* positionedAncestor = parentLayer->enclosingPositionedAncestor();
+            // to the positioned ancestor and subtract.
 
             LayoutPoint thisCoords;
-            layer->convertToLayerCoords(positionedAncestor, thisCoords);
+            layer->convertToLayerCoords(parentLayer, thisCoords);
 
             LayoutPoint ancestorCoords;
-            ancestorLayer->convertToLayerCoords(positionedAncestor, ancestorCoords);
+            ancestorLayer->convertToLayerCoords(parentLayer, ancestorCoords);
 
             location += (thisCoords - ancestorCoords);
             return ancestorLayer;
         }
     } else if (layoutObject->isColumnSpanAll()) {
         LayoutBlock* multicolContainer = layoutObject->containingBlock();
         ASSERT(toLayoutBlockFlow(multicolContainer)->multiColumnFlowThread());
         parentLayer = multicolContainer->layer();
         ASSERT(parentLayer);
     } else {
@@ skipping 1382 matching lines @@
 
 void showLayerTree(const blink::LayoutObject* layoutObject)
 {
     if (!layoutObject) {
         fprintf(stderr, "Cannot showLayerTree. Root is (nil)\n");
         return;
     }
     showLayerTree(layoutObject->enclosingLayer());
 }
 #endif