IntersectionObserver: compute correct intersection ratio for 0x0 elements

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

 // 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.
 
 #include "core/dom/IntersectionObservation.h"
 
 #include "core/dom/ElementRareData.h"
 #include "core/dom/IntersectionObserver.h"
 
 namespace blink {
@@ skipping 35 matching lines @@
   //   - Otherwise:
   //     - If root and target do not intersect, the threshold index is 0.
   //     - If root and target intersect but the intersection has zero-area
   //       (i.e., they have a coincident edge or corner), we consider the
   //       intersection to have "crossed" a zero threshold, but not crossed
   //       any non-zero threshold.
   unsigned newThresholdIndex;
   float newVisibleRatio = 0;
   if (geometry.targetRect().isEmpty()) {
     newThresholdIndex = geometry.doesIntersect() ? 1 : 0;
+    newVisibleRatio = geometry.doesIntersect() ? 1 : 0;

[szager1, 2016/12/09 17:30:07]

You also need to delete the preceding line, and instead use the same logic as
for the non-degenerate case to get newThresholdIndex (i.e.,
observer().firstThresholdGreaterThan(newVisibleRatio)).

[Sami, 2016/12/09 17:51:16]

Good catch. Rewrote this as we discussed offline.

   } else if (!geometry.doesIntersect()) {
     newThresholdIndex = 0;
   } else {
     float intersectionArea =
         geometry.intersectionRect().size().width().toFloat() *
         geometry.intersectionRect().size().height().toFloat();
     float targetArea = geometry.targetRect().size().width().toFloat() *
                        geometry.targetRect().size().height().toFloat();
     newVisibleRatio = intersectionArea / targetArea;
     newThresholdIndex = observer().firstThresholdGreaterThan(newVisibleRatio);
@@ skipping 21 matching lines @@
     target()->ensureIntersectionObserverData().removeObservation(observer());
   m_observer.clear();
 }
 
 DEFINE_TRACE(IntersectionObservation) {
   visitor->trace(m_observer);
   visitor->trace(m_target);
 }
 
 }  // namespace blink