[PE] Invalidate sticky constraints on composited scroll.

This ensures that property tree state for the sticky elements are updated
properly, to subsequently be used in hit testing and paint.

Bug:827224

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: If6fbfbe49b6b08046d40ecb2da636ee1f22cb486
Reviewed-on: https://chromium-review.googlesource.com/994462
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548120}

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

@@ -2122,6 +2122,11 @@ void PaintLayerScrollableArea::UpdateCompositingLayersAfterScroll() {
           kCompositingUpdateAfterGeometryChange);
     }
 
+    // Sticky constraints and paint property nodes need to be updated
+    // to the new sticky locations.
+    if (HasStickyDescendants())
+      InvalidateAllStickyConstraints();
+
     // If we have fixed elements and we scroll the root layer in RLS we might
     // change compositing since the fixed elements might now overlap a
     // composited layer.

third_party/WebKit/Source/core/paint/PaintLayerScrollableAreaTest.cpp

@@ -901,6 +901,37 @@ TEST_F(PaintLayerScrollableAreaTest, HitTestOverlayScrollbars) {
   EXPECT_EQ(hit_result.GetScrollbar(), scrollable_area->HorizontalScrollbar());
 }
 
+TEST_F(PaintLayerScrollableAreaTest, CompositedStickyDescendant) {
+  SetBodyInnerHTML(R"HTML(
+    <div id=scroller style="overflow: scroll; width: 500px; height: 300px;
+        will-change: transform">
+      <div id=sticky style="top: 0px; position: sticky; background: green">
+      </div>
+      <div style="width: 10px; height: 700px; background: lightblue"></div>
+    </div>
+  )HTML");
+  auto* scroller =
+      ToLayoutBoxModelObject(GetLayoutObjectByElementId("scroller"));
+  auto* scrollable_area = scroller->GetScrollableArea();
+  EXPECT_EQ(kPaintsIntoOwnBacking, scroller->Layer()->GetCompositingState());
+  auto* sticky = ToLayoutBoxModelObject(GetLayoutObjectByElementId("sticky"));
+
+  EXPECT_EQ(FloatSize(0, 0), sticky->FirstFragment()
+                                 .LocalBorderBoxProperties()
+                                 .Transform()
+                                 ->Matrix()
+                                 .To2DTranslation());
+
+  scrollable_area->SetScrollOffset(ScrollOffset(0, 50), kUserScroll);
+  GetDocument().View()->UpdateAllLifecyclePhases();
+
+  EXPECT_EQ(FloatSize(0, 50), sticky->FirstFragment()
+                                  .LocalBorderBoxProperties()
+                                  .Transform()
+                                  ->Matrix()
+                                  .To2DTranslation());
+}
+
 class NonRLSPaintLayerScrollableAreaTest
     : public PaintLayerScrollableAreaTest,
       private ScopedRootLayerScrollingForTest {