Add more specific metrics for main thread scrolling reasons

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

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights
  * reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@gmail.com>
@@ skipping 88 matching lines @@
       m_needsCompositedScrolling(false),
       m_rebuildHorizontalScrollbarLayer(false),
       m_rebuildVerticalScrollbarLayer(false),
       m_needsScrollOffsetClamp(false),
       m_needsRelayout(false),
       m_hadHorizontalScrollbarBeforeRelayout(false),
       m_hadVerticalScrollbarBeforeRelayout(false),
       m_scrollbarManager(*this),
       m_scrollCorner(nullptr),
       m_resizer(nullptr),
-      m_scrollAnchor(this)
+      m_scrollAnchor(this),
+      m_reasons(0)
 #if DCHECK_IS_ON()
       ,
       m_hasBeenDisposed(false)
 #endif
 {
   Node* node = box().node();
   if (node && node->isElementNode()) {
     // We save and restore only the scrollOffset as the other scroll values are
     // recalculated.
     Element* element = toElement(node);
@@ skipping 1612 matching lines @@
   bool backgroundSupportsLCDText =
       RuntimeEnabledFeatures::compositeOpaqueScrollersEnabled() &&
       layer->layoutObject()->style()->isStackingContext() &&
       layer->backgroundPaintLocation() & BackgroundPaintInScrollingContents &&
       layer->backgroundIsKnownToBeOpaqueInRect(
           toLayoutBox(layer->layoutObject())->paddingBoxRect()) &&
       !layer->compositesWithTransform() && !layer->compositesWithOpacity();
   if (mode == PaintLayerScrollableArea::ConsiderLCDText &&
       !layer->compositor()->preferCompositingToLCDTextEnabled() &&
       !backgroundSupportsLCDText)
     return false;

[bokan, 2016/12/19 14:54:25]

I'd prefer we call updateMainThreadScrollingReasonsFromStyle here. The reason
being, I'm a little worried about having it far away from the place where we
make the "real" decision about main thread scrolling - if someone updates the
conditions here, they'll likely forget to make a corresponding change below. If
the call is made here just before we return false, we're guaranteed to only set
the main thread reason if we're actually main thread scrolling.

You can merge this static method directly into
PLSA::updateNeedsCompositedScrolling. It'd also be better if you passed the
MainThreadScrollingReason from here into
updateMainThreadScrollingReasonsFromStyle rather than recomputing it there. e.g.

if (...) {
if (layer->compositesWithOpacity()) {
   updateMainThreadScrollingReasonsFromStyle(layer, MTSR::kHasOpacity,   
        true);
}
return false;
} else {
    updateMainThreadScrollingReasonsFromStyle(layer, MTSR::kHasOpacity,   
        false);
}

[yigu, 2016/12/19 18:55:04]

Since we clear all reasons for a layer at the beginning of this function, there
is no need to update with "false".

 
   // TODO(schenney) Tests fail if we do not also exclude
   // layer->layoutObject()->style()->hasBorderDecoration() (missing background
   // behind dashed borders). Resolve this case, or not, and update this check
   // with the results.
   return !(layer->size().isEmpty() || layer->hasDescendantWithClipPath() ||
            layer->hasAncestorWithClipPath() ||
            layer->layoutObject()->style()->hasBorderRadius());
 }
 
+// Check if current layer has layout object that contains properties that cause
+// main thread scrolling.
+void PaintLayerScrollableArea::updateMainThreadScrollingReasonsFromStyle(

[bokan, 2016/12/19 14:54:25]

Sorry to waffle on the name again, but after seeing it a bunch of times I think
updateStyleRelatedMainThreadScrollingReasons feels less awkward.

[yigu, 2016/12/19 18:55:04]

Done.

+    const PaintLayer* layer) const {
+  // If current layer has opacity and has not been recorded, add the layer into
+  // frameView and vice versa.
+  if (!layer || !layer->getScrollableArea())

[bokan, 2016/12/19 14:54:25]

Now that we're in PLSA you don't have to check for layer or scrollable area (you
can DCHECK(layer()))

[yigu, 2016/12/19 18:55:04]

Done.

+    return;
+
+  if (!box().frame()->page()->scrollingCoordinator())
+    return;
+  ScrollingCoordinator* scrollingCoordinator =
+      box().frame()->page()->scrollingCoordinator();
+  if (layer->compositesWithOpacity() !=
+      layer->getScrollableArea()->hasMainThreadScrollingReason(

[bokan, 2016/12/19 14:54:25]

layer->getScrollableArea() isn't needed anymore.

[yigu, 2016/12/19 18:55:04]

Done.

+          MainThreadScrollingReason::kHasOpacity)) {
+    layer->getScrollableArea()->flipMainThreadScrollingReason(
+        MainThreadScrollingReason::kHasOpacity);
+
+    scrollingCoordinator->adjustMainThreadReasonsFromStyle(
+        MainThreadScrollingReason::kHasOpacity, layer->compositesWithOpacity());
+  }
+}
+
 void PaintLayerScrollableArea::updateNeedsCompositedScrolling(
     LCDTextMode mode) {
   const bool needsCompositedScrolling =
       layerNeedsCompositedScrolling(mode, layer());
+  if (!needsCompositedScrolling)
+    updateMainThreadScrollingReasonsFromStyle(layer());
+
   if (static_cast<bool>(m_needsCompositedScrolling) !=
       needsCompositedScrolling) {
     m_needsCompositedScrolling = needsCompositedScrolling;
     layer()->didUpdateNeedsCompositedScrolling();
   }
 }
 
 void PaintLayerScrollableArea::setTopmostScrollChild(PaintLayer* scrollChild) {
   // We only want to track the topmost scroll child for scrollable areas with
   // overlay scrollbars.
@@ skipping 247 matching lines @@
 
 void PaintLayerScrollableArea::DelayScrollOffsetClampScope::
     clampScrollableAreas() {
   for (auto& scrollableArea : *s_needsClamp)
     scrollableArea->clampScrollOffsetAfterOverflowChange();
   delete s_needsClamp;
   s_needsClamp = nullptr;
 }
 
 }  // namespace blink