Fix bug in RenderLayer::scrollsWithRespectTo.

Source/core/rendering/RenderLayer.cpp

Index: Source/core/rendering/RenderLayer.cpp
diff --git a/Source/core/rendering/RenderLayer.cpp b/Source/core/rendering/RenderLayer.cpp
index 922e1a0ac76c8457a90283c6a378e9e52580d84b..18e6c98024aa4ffa81209a36960c7ae22485de67 100644
--- a/Source/core/rendering/RenderLayer.cpp
+++ b/Source/core/rendering/RenderLayer.cpp
@@ -416,6 +416,7 @@ bool RenderLayer::scrollsWithRespectTo(const RenderLayer* other) const
     const bool isRootFixedPos = position == FixedPosition && containingBlock->enclosingLayer() == rootLayer;
     const bool otherIsRootFixedPos = otherPosition == FixedPosition && otherContainingBlock->enclosingLayer() == rootLayer;
 
+    // FIXME: some of these cases don't look quite right.
     if (isRootFixedPos && otherIsRootFixedPos)
         return false;
     if (isRootFixedPos || otherIsRootFixedPos)
@@ -428,8 +429,13 @@ bool RenderLayer::scrollsWithRespectTo(const RenderLayer* other) const
     // closest scrollable ancestor.
     HashSet<const RenderObject*> containingBlocks;
     while (containingBlock) {
-        if (containingBlock->enclosingLayer()->scrollsOverflow())
+        if (containingBlock->enclosingLayer()->scrollsOverflow()) {
             break;
+        }
+        if (containingBlock->enclosingLayer() == other) {
+            // This layer does not scroll with respect to the other layer if the other one does not scroll and this one is a child.
+            return false;
+        }
         containingBlocks.add(containingBlock);
         containingBlock = containingBlock->containingBlock();
     }
@@ -437,9 +443,14 @@ bool RenderLayer::scrollsWithRespectTo(const RenderLayer* other) const
     // Do the same for the 2nd layer, but if we find a common containing block,
     // it means both layers are contained within a single non-scrolling subtree.
     // Hence, they will not scroll with respect to each other.
+    bool thisLayerScrollsOverflow = scrollsOverflow();
     while (otherContainingBlock) {
         if (containingBlocks.contains(otherContainingBlock))
             return false;
+        // The other layer scrolls with respect to this one if this one scrolls and it's a child.
+        if (!thisLayerScrollsOverflow && otherContainingBlock->enclosingLayer() == this)
+            return false;
+        // The other layer does not scroll with respect to this one if this one does not scroll and it's a child.
         if (otherContainingBlock->enclosingLayer()->scrollsOverflow())
             break;
         otherContainingBlock = otherContainingBlock->containingBlock();