Add more specific metrics for main thread scrolling reasons

third_party/WebKit/Source/core/layout/compositing/CompositingReasonFinder.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/CompositingReasonFinder.h"
 
 #include "core/CSSPropertyNames.h"
 #include "core/dom/Document.h"
 #include "core/frame/FrameView.h"
 #include "core/frame/Settings.h"
@@ skipping 160 matching lines @@
 }
 
 bool CompositingReasonFinder::requiresCompositingForAnimation(
     const ComputedStyle& style) const {
   if (style.subtreeWillChangeContents())
     return style.isRunningAnimationOnCompositor();
 
   return style.shouldCompositeForCurrentAnimations();
 }
 
+// Check if current layer has layout object that contains properties that cause
+// main thread scrolling.
+void CompositingReasonFinder::updateMainThreadScrollingReasonDueToLayoutObject(
+    const PaintLayer* layer) const {
+  // TODO(yigu) Keeping the reasons in PaintLayerScrollableArea
+  // is not ideal. When a layout becomes to non-scrollable

[bokan, 2016/12/13 14:38:57]

nit: "layout becomes to" -> "layer becomes"

[bokan, 2016/12/13 14:38:57]

Also, perhaps counterintuitively, I believe all layers created for a LayoutBox
will have a PLSA -- even if they're not scrollable. Perhaps you just need to
make the PLSA decrement any counts it has in dispose()

[yigu, 2016/12/13 20:54:10]

Done.

[yigu, 2016/12/13 20:54:10]

Done.

+  // the reasons kept in that scrollable area cannot be updated
+  // in the frameView correctly.
+  // If current layer has opacity and has not been recorded, add the layer into
+  // frameView and vice versa.
+  if (!layer || !layer->getScrollableArea())
+    return;
+  if (layer->compositesWithOpacity() !=
+      layer->getScrollableArea()->hasMainThreadScrollingReason(
+          MainThreadScrollingReason::kHasOpacity)) {
+    layer->getScrollableArea()->flipMainThreadScrollingReason(
+        MainThreadScrollingReason::kHasOpacity);
+    layer->layoutObject()
+        ->frameView()
+        ->adjustMainThreadReasonsDueToLayoutObject(
+            MainThreadScrollingReason::kHasOpacity,
+            layer->compositesWithOpacity());
+  }
+}
+
 bool CompositingReasonFinder::requiresCompositingForScrollDependentPosition(
     const PaintLayer* layer) const {
   if (layer->layoutObject()->style()->position() != FixedPosition &&
       layer->layoutObject()->style()->position() != StickyPosition)
     return false;
 
+  updateMainThreadScrollingReasonDueToLayoutObject(layer);

[bokan, 2016/12/13 14:38:57]

This is only meant for Fixed/Sticky layers? The name should make that clear.

[yigu, 2016/12/13 20:54:10]

No. It should be moved somewhere else.

+
   if (!(m_compositingTriggers & ViewportConstrainedPositionedTrigger) &&
       (!RuntimeEnabledFeatures::compositeOpaqueFixedPositionEnabled() ||
        !layer->backgroundIsKnownToBeOpaqueInRect(
            LayoutRect(layer->boundingBoxForCompositing())) ||
        layer->compositesWithTransform() || layer->compositesWithOpacity())) {
     return false;
   }
   // Don't promote fixed position elements that are descendants of a non-view
   // container, e.g. transformed elements.  They will stay fixed wrt the
   // container rather than the enclosing frame.
   if (layer->sticksToViewport())
     return m_layoutView.frameView()->isScrollable();
   return layer->layoutObject()->style()->position() == StickyPosition &&
          layer->ancestorOverflowLayer()->scrollsOverflow();
 }
 
 }  // namespace blink