Update comments about not setting kHandlingScrollFromMainThread on subframes

third_party/WebKit/Source/core/frame/FrameView.cpp

 /*
  * Copyright (C) 1998, 1999 Torben Weis <weis@kde.org>
  *                     1999 Lars Knoll <knoll@kde.org>
  *                     1999 Antti Koivisto <koivisto@kde.org>
  *                     2000 Dirk Mueller <mueller@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  *           (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
@@ skipping 4780 matching lines @@
   if (WebLayer* scrollLayer =
           toLocalFrame(frame).view()->layerForScrolling()->platformLayer()) {
     if (reasons) {
       scrollLayer->addMainThreadScrollingReasons(reasons);
     } else {
       // Clear all main thread scrolling reasons except the one that's set
       // if there is a running scroll animation.
       scrollLayer->clearMainThreadScrollingReasons(
           ~MainThreadScrollingReason::kHandlingScrollFromMainThread);
     }
+    reasons |= scrollLayer->mainThreadScrollingReasons();

[pdr., 2017/01/06 20:41:29]

Will updateSubFrameScrollOnMainReason always get called after
ScrollAnimator::addMainThreadScrollingReason and
ScrollAnimator::removeMainThreadScrollingReason? Can you add a unit test for
both of these cases?

kHandlingScrollFromMainThread is different from the other main thread scrolling
reasons. Could we unify them so all updates go through the same codepath?

[yigu, 2017/01/11 22:44:45]

As discussed, we will not update this reason on subframes.

   }
 
   Frame* child = frame.tree().firstChild();
   while (child) {
     updateSubFrameScrollOnMainReason(*child, reasons);
     child = child->tree().nextSibling();
   }
 
   if (frame.isMainFrame())
     m_mainThreadScrollingReasons = reasons;
@@ skipping 93 matching lines @@
        reason < MainThreadScrollingReason::kMainThreadScrollingReasonCount;
        ++reason) {
     if (m_mainThreadScrollingReasonsCounter[reason] > 0) {
       reasons |= 1 << (reason - 1);
     }
   }
   return reasons;
 }
 
 }  // namespace blink