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

third_party/WebKit/Source/core/paint/PaintLayer.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>
@@ skipping 52 matching lines @@
 #include "core/layout/LayoutView.h"
 #include "core/layout/api/LayoutPartItem.h"
 #include "core/layout/api/LayoutViewItem.h"
 #include "core/layout/compositing/CompositedLayerMapping.h"
 #include "core/layout/compositing/PaintLayerCompositor.h"
 #include "core/layout/svg/LayoutSVGResourceClipper.h"
 #include "core/layout/svg/LayoutSVGRoot.h"
 #include "core/page/Page.h"
 #include "core/page/scrolling/RootScrollerController.h"
 #include "core/page/scrolling/ScrollingCoordinator.h"
+#include "core/page/scrolling/StickyPositionScrollingConstraints.h"
 #include "core/paint/BoxReflectionUtils.h"
 #include "core/paint/FilterEffectBuilder.h"
 #include "core/paint/ObjectPaintInvalidator.h"
 #include "platform/LengthFunctions.h"
 #include "platform/RuntimeEnabledFeatures.h"
 #include "platform/geometry/FloatPoint3D.h"
 #include "platform/geometry/FloatRect.h"
 #include "platform/geometry/TransformState.h"
 #include "platform/graphics/CompositorFilterOperations.h"
 #include "platform/graphics/filters/Filter.h"
@@ skipping 239 matching lines @@
     // a self-painting descendant in this case, there is no need to dirty our
     // ancestors further.
     if (layer->isSelfPaintingLayer()) {
       DCHECK(!parent() || parent()->m_hasSelfPaintingLayerDescendantDirty ||
              parent()->m_hasSelfPaintingLayerDescendant);
       break;
     }
   }
 }
 
-bool PaintLayer::sticksToViewport() const {
-  if (layoutObject().style()->position() != EPosition::kFixed &&
-      layoutObject().style()->position() != EPosition::kSticky)
+bool PaintLayer::sticksToScroller() const {
+  return layoutObject().style()->position() == EPosition::kSticky &&
+         ancestorOverflowLayer()
+             ->getScrollableArea()
+             ->stickyConstraintsMap()
+             .at(const_cast<PaintLayer*>(this))
+             .anchorEdges();
+}
+
+bool PaintLayer::fixedToViewport() const {
+  if (layoutObject().style()->position() != EPosition::kFixed)
     return false;
 
   // TODO(pdr): This approach of calculating the nearest scroll node is O(n).
   // An option for improving this is to cache the nearest scroll node in
   // the local border box properties.
   if (RuntimeEnabledFeatures::slimmingPaintV2Enabled()) {
     const auto* viewProperties = layoutObject().view()->paintProperties();
     const ScrollPaintPropertyNode* ancestorTargetScrollNode;
-    if (layoutObject().style()->position() == EPosition::kFixed) {
-      ancestorTargetScrollNode = viewProperties->localBorderBoxProperties()
-                                     ->transform()
-                                     ->findEnclosingScrollNode();
-    } else {
-      ancestorTargetScrollNode = viewProperties->contentsProperties()
-                                     ->transform()
-                                     ->findEnclosingScrollNode();

[flackr, 2017/02/24 22:59:44]

Do we still need to something like this for sticksToScroller?

-    }
+    ancestorTargetScrollNode = viewProperties->localBorderBoxProperties()
+                                   ->transform()
+                                   ->findEnclosingScrollNode();
 
     const auto* properties = layoutObject().paintProperties();
     const auto* transform = properties->localBorderBoxProperties()->transform();
     return transform->findEnclosingScrollNode() == ancestorTargetScrollNode;
   }
 
-  return (layoutObject().style()->position() == EPosition::kFixed &&
-          layoutObject().containerForFixedPosition() ==
-              layoutObject().view()) ||
-         (layoutObject().style()->position() == EPosition::kSticky &&
-          (!ancestorScrollingLayer() || ancestorScrollingLayer() == root()));

[flackr, 2017/02/24 22:59:44]

ancestorScrollingLayer was not the correct "scroller" for sticky because it
excludes non-visible overflow which isn't actually scrollable.

+  return layoutObject().containerForFixedPosition() == layoutObject().view();
 }
 
 bool PaintLayer::scrollsWithRespectTo(const PaintLayer* other) const {
-  if (sticksToViewport() != other->sticksToViewport())
+  if (fixedToViewport() != other->fixedToViewport())
+    return true;
+  // If either element sticks we cannot trivially determine that the layers do
+  // not scroll with respect to each other.
+  if (sticksToScroller() || other->sticksToScroller())
     return true;
   return ancestorScrollingLayer() != other->ancestorScrollingLayer();
 }
 
 void PaintLayer::updateLayerPositionsAfterOverflowScroll() {
   clipper(PaintLayer::DoNotUseGeometryMapper)
       .clearClipRectsIncludingDescendants();
   updateLayerPositionRecursive();
 }
 
@@ skipping 2850 matching lines @@
 }
 
 void showLayerTree(const blink::LayoutObject* layoutObject) {
   if (!layoutObject) {
     LOG(INFO) << "Cannot showLayerTree. Root is (nil)";
     return;
   }
   showLayerTree(layoutObject->enclosingLayer());
 }
 #endif