Do not promote position sticky or fixed elements unless they move with scroll.

third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp

Index: third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp b/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
index 1076826b28bd9a6aa4651db6a5eb340c0e3a0ee0..f7bed6a20d8c98621454d9126cb9e0fe4e395b75 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
@@ -74,13 +74,13 @@ LayoutBoxModelObject* findFirstStickyBetween(LayoutObject* from,
                                              LayoutObject* to) {
   LayoutObject* maybeStickyAncestor = from;
   while (maybeStickyAncestor && maybeStickyAncestor != to) {
-    if (maybeStickyAncestor->isStickyPositioned()) {
+    if (maybeStickyAncestor->style()->hasStickyConstrainedPosition()) {
       return toLayoutBoxModelObject(maybeStickyAncestor);
     }
 
     maybeStickyAncestor =
         maybeStickyAncestor->isLayoutInline()
-            ? maybeStickyAncestor->containingBlock()
+            ? maybeStickyAncestor->container()

[flackr, 2017/03/20 21:04:59]

Stephen I noticed when I added an unanchored sticky parent that if you had
deeply nested inlines this code used to skip straight past the first parent to
the containing block.

[smcgruer, 2017/03/21 14:06:19]

Good catch, this should not have been jumping to the containing block. Can you
add a test for exactly that scenario? (I.e. multiple inline elements between the
inline sticky ancestor and the current sticky)?

[chrishtr, 2017/03/21 16:43:16]

container() is the containing block. containingBlock() is something slightly
different.

[flackr, 2017/03/21 16:56:32]

FYI, container() isn't equivalent to CSS containing block. It returns parent()
for non absolute / fixed, but CSS defines the containing block to be the nearest
block container parent (i.e. skipping inlines)
https://www.w3.org/TR/CSS2/visudet.html#containing-block-details.

             : toLayoutBox(maybeStickyAncestor)->locationContainer();
   }
   return nullptr;
@@ -249,7 +249,8 @@ void LayoutBoxModelObject::willBeDestroyed() {
     // 0 during destruction.
     if (LocalFrame* frame = this->frame()) {
       if (FrameView* frameView = frame->view()) {
-        if (style()->hasViewportConstrainedPosition())
+        if (style()->hasViewportConstrainedPosition() ||
+            style()->hasStickyConstrainedPosition())
           frameView->removeViewportConstrainedObject(*this);
       }
     }
@@ -430,9 +431,9 @@ void LayoutBoxModelObject::styleDidChange(StyleDifference diff,
         style()->position() == EPosition::kFixed;
     bool oldStyleIsViewportConstrained =
         oldStyle && oldStyle->position() == EPosition::kFixed;
-    bool newStyleIsSticky = style()->position() == EPosition::kSticky;
+    bool newStyleIsSticky = style()->hasStickyConstrainedPosition();
     bool oldStyleIsSticky =
-        oldStyle && oldStyle->position() == EPosition::kSticky;
+        oldStyle && oldStyle->hasStickyConstrainedPosition();
 
     if (newStyleIsSticky != oldStyleIsSticky) {
       if (newStyleIsSticky) {
@@ -1053,6 +1054,8 @@ void LayoutBoxModelObject::updateStickyPositionConstraints() const {
     constraints.addAnchorEdge(
         StickyPositionScrollingConstraints::AnchorEdgeBottom);
   }
+  // At least one edge should be anchored if we are calculating constraints.
+  DCHECK(constraints.anchorEdges());
   scrollableArea->stickyConstraintsMap().set(layer(), constraints);
 }