Only automatically promote sticky position elements which move with scroll.

third_party/WebKit/Source/core/layout/compositing/CompositedLayerMappingTest.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/CompositedLayerMapping.h"
 
 #include "core/frame/FrameView.h"
 #include "core/layout/LayoutBoxModelObject.h"
 #include "core/layout/LayoutTestHelper.h"
 #include "core/layout/api/LayoutViewItem.h"
@@ skipping 1627 matching lines @@
   EXPECT_FALSE(stickyLayer->needsCompositingInputsUpdate());
 
   constraint = stickyMapping->mainGraphicsLayer()
                    ->contentLayer()
                    ->layer()
                    ->stickyPositionConstraint();
   EXPECT_EQ(IntPoint(0, 10),
             IntPoint(constraint.parentRelativeStickyBoxOffset));
 }
 
+TEST_P(CompositedLayerMappingTest, StickyPositionNotSquashed) {
+  setBodyInnerHTML(
+      "<style>"
+      "#scroller { overflow: auto; height: 200px; }"
+      "#sticky1, #sticky2, #sticky3 {position: sticky; top: 0; width: 50px;"
+      "    height: 50px; background: rgba(0, 128, 0, 0.5);}"
+      "#sticky1 {backface-visibility: hidden;}"
+      ".spacer {height: 2000px;}"
+      "</style>"
+      "<div id='scroller'>"
+      "  <div id='sticky1'></div>"
+      "  <div id='sticky2'></div>"
+      "  <div id='sticky3'></div>"
+      "  <div class='spacer'></div>"
+      "</div>");
+
+  PaintLayer* sticky1 =
+      toLayoutBlock(getLayoutObjectByElementId("sticky1"))->layer();
+  PaintLayer* sticky2 =
+      toLayoutBlock(getLayoutObjectByElementId("sticky2"))->layer();
+  PaintLayer* sticky3 =
+      toLayoutBlock(getLayoutObjectByElementId("sticky3"))->layer();
+  EXPECT_EQ(PaintsIntoOwnBacking, sticky1->compositingState());
+  EXPECT_EQ(NotComposited, sticky2->compositingState());
+  EXPECT_EQ(NotComposited, sticky3->compositingState());
+
+  PaintLayer* scroller =
+      toLayoutBlock(getLayoutObjectByElementId("scroller"))->layer();
+  PaintLayerScrollableArea* scrollableArea = scroller->getScrollableArea();
+  scrollableArea->scrollToAbsolutePosition(
+      FloatPoint(scrollableArea->scrollPosition().y(), 100));
+  document().view()->updateAllLifecyclePhases();
+
+  // Now that sticky2 and sticky3 overlap sticky1 they will be promoted, but
+  // they should not be squashed into the same layer because they scroll with
+  // respect to each other.
+  EXPECT_EQ(PaintsIntoOwnBacking, sticky1->compositingState());
+  EXPECT_EQ(PaintsIntoOwnBacking, sticky2->compositingState());
+  EXPECT_EQ(PaintsIntoOwnBacking, sticky3->compositingState());
+}
+
 TEST_P(CompositedLayerMappingTest, StickyPositionNestedStickyContentOffset) {
   setBodyInnerHTML(
       "<style>.composited { will-change: transform; }"
       "#scroller { overflow: auto; height: 200px; width: 200px; }"
       ".container { height: 500px; }"
       "#outerSticky { position: sticky; top: 0; height: 100px; }"
       "#middleSticky { position: sticky; top: 10px; height: 50px; }"
       "#innerSticky { position: sticky; top: 25px; height: 25px; }</style>"
       "<div id='scroller' class='composited'>"
       "  <div style='height: 50px'></div>"
@@ skipping 75 matching lines @@
   // the parent layer should remain constant.
   EXPECT_EQ(IntPoint(0, 10),
             IntPoint(outerStickyConstraint.parentRelativeStickyBoxOffset));
   EXPECT_EQ(IntPoint(0, 0),
             IntPoint(middleStickyConstraint.parentRelativeStickyBoxOffset));
   EXPECT_EQ(IntPoint(0, 5),
             IntPoint(innerStickyConstraint.parentRelativeStickyBoxOffset));
 }
 
 }  // namespace blink