IntersectionObserver: Always send an initial notification.

third_party/WebKit/Source/core/dom/IntersectionObservation.h

 // Copyright 2016 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.
 
 #ifndef IntersectionObservation_h
 #define IntersectionObservation_h
 
 #include "core/dom/DOMHighResTimeStamp.h"
 #include "platform/heap/Handle.h"
 
 namespace blink {
 
 class Element;
 class IntersectionObserver;
 
 class IntersectionObservation final
     : public GarbageCollected<IntersectionObservation> {
  public:
   IntersectionObservation(IntersectionObserver&,
                           Element&,
                           bool shouldReportRootBounds);
 
   IntersectionObserver* observer() const { return m_observer.get(); }
   Element* target() const { return m_target; }
   unsigned lastThresholdIndex() const { return m_lastThresholdIndex; }
-  void setLastThresholdIndex(unsigned index) { m_lastThresholdIndex = index; }
+  void setLastThresholdIndex(int index) { m_lastThresholdIndex = index; }

[ojan, 2017/01/26 01:44:56]

I assume this is leftover from a previous iteration of the patch?

[szager1, 2017/01/30 19:37:41]

Yep, thanks for the catch.

   void computeIntersectionObservations(DOMHighResTimeStamp);
   void disconnect();
 
   DECLARE_TRACE();
 
  private:
   Member<IntersectionObserver> m_observer;
   WeakMember<Element> m_target;
 
   const unsigned m_shouldReportRootBounds : 1;
+
+  // HACK: the spec calls for the inital value of m_lastThresholdIndex to be
+  // -1.  But if m_lastThresholdIndex is changed to a signed int type, then
+  // we can't bit-pack it.  Instead, inflate all thresholdIndex values by 1.

[ojan, 2017/01/26 01:44:56]

Since we're going hacky, how about initializing m_lastThresholdIndex to
0x40000000 and leaving the rest of the code untouched? Then the hack is
localized to one place and there's no need for a RELEASE_ASSERT.

[szager1, 2017/01/30 19:37:41]

The RELEASE_ASSERT is me being paranoid.  This patch doesn't make it any more
necessary than it was before; it just occurred to me that guarding against
overflow is probably a good idea.

0x40000000 doesn't fit in 30 bits.  I could initialize it to
kMaxThresholdIndex-1, if you think that's clearer.

   unsigned m_lastThresholdIndex : 30;
+  static const unsigned kMaxThresholdIndex = (unsigned)0x40000000;
 };
 
 }  // namespace blink
 
 #endif  // IntersectionObservation_h