Create scroll properties for main thread scrolling even w/o scrolling

If an object with overflow scrolling does not actually scroll, we would not
create a scroll paint property node before this patch. This was wrong because
a paint property node may be needed for main thread scrolling reasons.

BUG=671419
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2553093003
Cr-Commit-Position: refs/heads/master@{#436833}

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

@@ -640,15 +640,34 @@ void PaintPropertyTreeBuilder::updateSvgLocalToBorderBoxTransform(
   context.current.paintOffset = LayoutPoint();
 }
 
+MainThreadScrollingReasons mainScrollingReasons(const LayoutObject& object) {
+  MainThreadScrollingReasons reasons = 0;
+  if (!object.document().settings()->threadedScrollingEnabled())
+    reasons |= MainThreadScrollingReason::kThreadedScrollingDisabled;
+  // Checking for descendants in the layout tree has two downsides:
+  // 1) There can be more descendants in layout order than in paint order (e.g.,
+  //    fixed position objects).
+  // 2) Iterating overall all background attachment fixed objects for every
+  //    scroll node can be slow, though there will be none in the common case.
+  const FrameView& frameView = *object.frameView();
+  if (frameView.hasBackgroundAttachmentFixedDescendants(object))
+    reasons |= MainThreadScrollingReason::kHasBackgroundAttachmentFixedObjects;
+  return reasons;
+}
+
 void PaintPropertyTreeBuilder::updateScrollAndScrollTranslation(
     const LayoutObject& object,
     PaintPropertyTreeBuilderContext& context) {
   if (object.needsPaintPropertyUpdate() || context.forceSubtreeUpdate) {
+    bool needsScrollProperties = false;
     if (object.hasOverflowClip()) {
+      auto mainThreadScrollingReasons = mainScrollingReasons(object);
       const LayoutBox& box = toLayoutBox(object);
-      const PaintLayerScrollableArea* scrollableArea = box.getScrollableArea();
+      const auto* scrollableArea = box.getScrollableArea();
       IntSize scrollOffset = box.scrolledContentOffset();
-      if (!scrollOffset.isZero() || scrollableArea->scrollsOverflow()) {
+      if (mainThreadScrollingReasons || !scrollOffset.isZero() ||
+          scrollableArea->scrollsOverflow()) {
+        needsScrollProperties = true;
         auto& properties =
             object.getMutableForPainting().ensurePaintProperties();
         TransformationMatrix matrix = TransformationMatrix().translate(
@@ -664,32 +683,18 @@ void PaintPropertyTreeBuilder::updateScrollAndScrollTranslation(
             scrollableArea->userInputScrollable(HorizontalScrollbar);
         bool userScrollableVertical =
             scrollableArea->userInputScrollable(VerticalScrollbar);
-        MainThreadScrollingReasons reasons = 0;
-        if (!object.document().settings()->threadedScrollingEnabled())
-          reasons |= MainThreadScrollingReason::kThreadedScrollingDisabled;
-        // Checking for descendants in the layout tree has two downsides:
-        // 1) There can be more descendants in layout order than in paint
-        //    order (e.g., fixed position objects).
-        // 2) Iterating overall all background attachment fixed objects for
-        //    every scroll node can be slow, though there will be no objects
-        //    in the common case.
-        const FrameView& frameView = *object.frameView();
-        if (frameView.hasBackgroundAttachmentFixedDescendants(object)) {
-          reasons |=
-              MainThreadScrollingReason::kHasBackgroundAttachmentFixedObjects;
-        }
         context.forceSubtreeUpdate |= properties.updateScroll(
             context.current.scroll, properties.scrollTranslation(), scrollClip,
             scrollBounds, userScrollableHorizontal, userScrollableVertical,
-            reasons);
-      } else {
-        // Ensure pre-existing properties are cleared when there is no
-        // scrolling.
-        auto* properties = object.getMutableForPainting().paintProperties();
-        if (properties) {
-          context.forceSubtreeUpdate |= properties->clearScrollTranslation();
-          context.forceSubtreeUpdate |= properties->clearScroll();
-        }
+            mainThreadScrollingReasons);
+      }
+    }
+
+    if (!needsScrollProperties) {
+      // Ensure pre-existing properties are cleared.
+      if (auto* properties = object.getMutableForPainting().paintProperties()) {
+        context.forceSubtreeUpdate |= properties->clearScrollTranslation();
+        context.forceSubtreeUpdate |= properties->clearScroll();
       }
     }
   }

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

@@ -2696,6 +2696,36 @@ TEST_P(PaintPropertyTreeBuilderTest,
                    ->hasBackgroundAttachmentFixedDescendants());
 }
 
+TEST_P(PaintPropertyTreeBuilderTest, MainThreadScrollReasonsWithoutScrolling) {
+  setBodyInnerHTML(
+      "<style>"
+      "  #overflow {"
+      "    overflow: scroll;"
+      "    width: 100px;"
+      "    height: 100px;"
+      "  }"
+      "  .backgroundAttachmentFixed {"
+      "    background-image: url('foo');"
+      "    background-attachment: fixed;"
+      "    width: 10px;"
+      "    height: 10px;"
+      "  }"
+      "  .forceScroll {"
+      "    height: 4000px;"
+      "  }"
+      "</style>"
+      "<div id='overflow'>"
+      "  <div class='backgroundAttachmentFixed'></div>"
+      "</div>"
+      "<div class='forceScroll'></div>");
+  Element* overflow = document().getElementById("overflow");
+  EXPECT_TRUE(frameScroll()->hasBackgroundAttachmentFixedDescendants());
+  EXPECT_TRUE(overflow->layoutObject()
+                  ->paintProperties()
+                  ->scroll()
+                  ->hasBackgroundAttachmentFixedDescendants());
+}
+
 TEST_P(PaintPropertyTreeBuilderTest,
        BackgroundAttachmentFixedMainThreadScrollReasonsWithFixedScroller) {
   setBodyInnerHTML(