Only create sticky position constraints for constrained sticky position.

third_party/WebKit/Source/core/layout/compositing/CompositingInputsUpdater.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/layout/compositing/CompositingInputsUpdater.h"
 
 #include "core/dom/Document.h"
 #include "core/frame/FrameHost.h"
 #include "core/frame/FrameView.h"
 #include "core/layout/LayoutBlock.h"
@@ skipping 83 matching lines @@
 
 void CompositingInputsUpdater::updateRecursive(PaintLayer* layer,
                                                UpdateType updateType,
                                                AncestorInfo info) {
   if (!layer->childNeedsCompositingInputsUpdate() && updateType != ForceUpdate)
     return;
 
   const PaintLayer* previousOverflowLayer = layer->ancestorOverflowLayer();
   layer->updateAncestorOverflowLayer(info.lastOverflowClipLayer);
   if (info.lastOverflowClipLayer && layer->needsCompositingInputsUpdate() &&
-      layer->layoutObject().style()->position() == EPosition::kSticky) {
+      layer->layoutObject().style()->hasStickyConstrainedPosition()) {

[chrishtr, 2017/03/27 16:28:07]

Which code clears out the sticky position constraints when it becomes
unconstrained?

[flackr, 2017/04/07 18:15:45]

LayoutBoxModelObject::styleDidChange checks if the style became constrained or
no longer constrained and in the latter case removes the constraints from the
layer.

     if (!RuntimeEnabledFeatures::rootLayerScrollingEnabled()) {
       if (info.lastOverflowClipLayer != previousOverflowLayer) {
         // Old ancestor scroller should no longer have these constraints.
         DCHECK(!previousOverflowLayer ||
                !previousOverflowLayer->getScrollableArea()
                     ->stickyConstraintsMap()
                     .contains(layer));
 
         // If our ancestor scroller has changed and the previous one was the
         // root layer, we are no longer viewport constrained.
@@ skipping 157 matching lines @@
   ASSERT(!layer->needsCompositingInputsUpdate());
 
   for (PaintLayer* child = layer->firstChild(); child;
        child = child->nextSibling())
     assertNeedsCompositingInputsUpdateBitsCleared(child);
 }
 
 #endif
 
 }  // namespace blink