Handle nested position:sticky elements

third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.cpp

Index: third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.cpp
diff --git a/third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.cpp b/third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.cpp
index d0c2d01708f9e499bcf3344607592d95753dd366..66037232f50fe03a1ce29101430a61dc5556f44c 100644
--- a/third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.cpp
+++ b/third_party/WebKit/Source/core/page/scrolling/StickyPositionScrollingConstraints.cpp
@@ -7,16 +7,34 @@
 namespace blink {
 
 FloatSize StickyPositionScrollingConstraints::computeStickyOffset(
-    const FloatRect& viewportRect) const {
-  FloatRect boxRect = m_scrollContainerRelativeStickyBoxRect;
+    const FloatRect& viewportRect,
+    StickyPositionScrollingConstraints* ancestorStickyBoxConstraints,
+    StickyPositionScrollingConstraints* ancestorContainingBlockConstraints) {
+  FloatSize ancestorStickyBoxOffset =
+      ancestorStickyBoxConstraints
+          ? ancestorStickyBoxConstraints->getTotalStickyBoxStickyOffset()
+          : FloatSize();
+  FloatSize ancestorContainingBlockOffset =
+      ancestorContainingBlockConstraints
+          ? ancestorContainingBlockConstraints
+                ->getTotalContainingBlockStickyOffset()
+          : FloatSize();
+
+  // Adjust the constraint rect locations based on our ancestor sticky elements,
+  // if they exist. These adjustments are necessary to avoid double offsetting
+  // in the case of nested sticky elements.
+  FloatRect stickyBoxRect = m_scrollContainerRelativeStickyBoxRect;
+  FloatRect containingBlockRect = m_scrollContainerRelativeContainingBlockRect;
+  stickyBoxRect.move(ancestorStickyBoxOffset + ancestorContainingBlockOffset);
+  containingBlockRect.move(ancestorContainingBlockOffset);
+
+  FloatRect boxRect = stickyBoxRect;
 
   if (hasAnchorEdge(AnchorEdgeRight)) {
     float rightLimit = viewportRect.maxX() - m_rightOffset;
-    float rightDelta = std::min<float>(
-        0, rightLimit - m_scrollContainerRelativeStickyBoxRect.maxX());
+    float rightDelta = std::min<float>(0, rightLimit - stickyBoxRect.maxX());
     float availableSpace =
-        std::min<float>(0, m_scrollContainerRelativeContainingBlockRect.x() -
-                               m_scrollContainerRelativeStickyBoxRect.x());
+        std::min<float>(0, containingBlockRect.x() - stickyBoxRect.x());
     if (rightDelta < availableSpace)
       rightDelta = availableSpace;
 
@@ -25,11 +43,9 @@ FloatSize StickyPositionScrollingConstraints::computeStickyOffset(
 
   if (hasAnchorEdge(AnchorEdgeLeft)) {
     float leftLimit = viewportRect.x() + m_leftOffset;
-    float leftDelta = std::max<float>(
-        0, leftLimit - m_scrollContainerRelativeStickyBoxRect.x());
+    float leftDelta = std::max<float>(0, leftLimit - stickyBoxRect.x());
     float availableSpace =
-        std::max<float>(0, m_scrollContainerRelativeContainingBlockRect.maxX() -
-                               m_scrollContainerRelativeStickyBoxRect.maxX());
+        std::max<float>(0, containingBlockRect.maxX() - stickyBoxRect.maxX());
     if (leftDelta > availableSpace)
       leftDelta = availableSpace;
 
@@ -38,11 +54,9 @@ FloatSize StickyPositionScrollingConstraints::computeStickyOffset(
 
   if (hasAnchorEdge(AnchorEdgeBottom)) {
     float bottomLimit = viewportRect.maxY() - m_bottomOffset;
-    float bottomDelta = std::min<float>(
-        0, bottomLimit - m_scrollContainerRelativeStickyBoxRect.maxY());
+    float bottomDelta = std::min<float>(0, bottomLimit - stickyBoxRect.maxY());
     float availableSpace =
-        std::min<float>(0, m_scrollContainerRelativeContainingBlockRect.y() -
-                               m_scrollContainerRelativeStickyBoxRect.y());
+        std::min<float>(0, containingBlockRect.y() - stickyBoxRect.y());
     if (bottomDelta < availableSpace)
       bottomDelta = availableSpace;
 
@@ -51,18 +65,25 @@ FloatSize StickyPositionScrollingConstraints::computeStickyOffset(
 
   if (hasAnchorEdge(AnchorEdgeTop)) {
     float topLimit = viewportRect.y() + m_topOffset;
-    float topDelta = std::max<float>(
-        0, topLimit - m_scrollContainerRelativeStickyBoxRect.y());
+    float topDelta = std::max<float>(0, topLimit - stickyBoxRect.y());
     float availableSpace =
-        std::max<float>(0, m_scrollContainerRelativeContainingBlockRect.maxY() -
-                               m_scrollContainerRelativeStickyBoxRect.maxY());
+        std::max<float>(0, containingBlockRect.maxY() - stickyBoxRect.maxY());
+
     if (topDelta > availableSpace)
       topDelta = availableSpace;
 
     boxRect.move(0, topDelta);
   }
 
-  return boxRect.location() - m_scrollContainerRelativeStickyBoxRect.location();
+  FloatSize stickyOffset = boxRect.location() - stickyBoxRect.location();
+
+  m_totalStickyBoxStickyOffset = ancestorStickyBoxOffset + stickyOffset;
+  m_totalContainingBlockStickyOffset =
+      ancestorStickyBoxOffset + ancestorContainingBlockOffset + stickyOffset;
+
+  m_cachedComputedStickyOffset = stickyOffset;
+
+  return stickyOffset;
 }
 
 }  // namespace blink