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 0507c39044c21b83f227d51c5649d5bdfa0a6e7d..04499a19bb9fc83ed63374cc4d1eafdc5cd98d7f 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
@@ -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())

[chrishtr, 2017/03/16 00:24:03]

This was a bug before?

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

Not so much a bug, just hasStickyConstrainedPosition now also checks that it is
actually constrained by one of top/bottom/left/right. If it's not we should
treat position sticky as if it is position relative (i.e. not cause slow
scrolling, not promote it, etc). I realized this also needs to be updated in
CompositingInputsUpdater, so I've updated it there and added some tests.
Interestingly, as far as I can tell the SPv2 version never adds sticky position
to the frameview viewport constrained set (but with root layer scrolling we
don't need this anymore).

           frameView->removeViewportConstrainedObject(*this);
       }
     }