Bug 1611204 - Fix IntersectionObserverEntry.isIntersecting to match other browsers. r=mstange

Note that no browser matches the spec (see
w3c/IntersectionObserver#432), but that our
behavior is reasonably close to them. So do this to match them.

Differential Revision: https://phabricator.services.mozilla.com/D76603

Author: emilio

dom/base/DOMIntersectionObserver.cpp

@@ -618,7 +618,9 @@ void DOMIntersectionObserver::Update(Document* aDocument,
     // length of observer.thresholds if intersectionRatio is greater than or
     // equal to the last entry in observer.thresholds.
     int32_t thresholdIndex = -1;
-    // FIXME(emilio): Why the isIntersecting check?
+
+    // If not intersecting, we can just shortcut, as we know that the thresholds
+    // are always between 0 and 1.
     if (isIntersecting) {
       thresholdIndex = mThresholds.IndexOfFirstElementGt(intersectionRatio);
       if (thresholdIndex == 0) {
@@ -628,26 +630,30 @@ void DOMIntersectionObserver::Update(Document* aDocument,
         // neither Chrome nor the WPT tests expect this behavior, so treat these
         // two cases as one.
         //
-        // FIXME(emilio): Looks like a good candidate for a spec issue.
+        // See https://github.com/w3c/IntersectionObserver/issues/432 about
+        // this.
         thresholdIndex = -1;
       }
     }
 
     // Steps 2.10 - 2.15.
     if (target->UpdateIntersectionObservation(this, thresholdIndex)) {
+      // See https://github.com/w3c/IntersectionObserver/issues/432 about
+      // why we use thresholdIndex > 0 rather than isIntersecting for the
+      // entry's isIntersecting value.
       QueueIntersectionObserverEntry(
           target, time,
           origin == BrowsingContextOrigin::Similar ? Some(rootBounds)
                                                    : Nothing(),
-          targetRect, intersectionRect, intersectionRatio);
+          targetRect, intersectionRect, thresholdIndex > 0, intersectionRatio);
     }
   }
 }
 
 void DOMIntersectionObserver::QueueIntersectionObserverEntry(
     Element* aTarget, DOMHighResTimeStamp time, const Maybe<nsRect>& aRootRect,
     const nsRect& aTargetRect, const Maybe<nsRect>& aIntersectionRect,
-    double aIntersectionRatio) {
+    bool aIsIntersecting, double aIntersectionRatio) {
   RefPtr<DOMRect> rootBounds;
   if (aRootRect.isSome()) {
     rootBounds = new DOMRect(this);
@@ -661,7 +667,7 @@ void DOMIntersectionObserver::QueueIntersectionObserverEntry(
   }
   RefPtr<DOMIntersectionObserverEntry> entry = new DOMIntersectionObserverEntry(
       this, time, rootBounds.forget(), boundingClientRect.forget(),
-      intersectionRect.forget(), aIntersectionRect.isSome(), aTarget,
+      intersectionRect.forget(), aIsIntersecting, aTarget,
       aIntersectionRatio);
   mQueuedEntries.AppendElement(entry.forget());
 }

dom/base/DOMIntersectionObserver.h

@@ -31,9 +31,9 @@ class DOMIntersectionObserverEntry final : public nsISupports,
                                double aIntersectionRatio)
       : mOwner(aOwner),
         mTime(aTime),
-        mRootBounds(aRootBounds),
-        mBoundingClientRect(aBoundingClientRect),
-        mIntersectionRect(aIntersectionRect),
+        mRootBounds(std::move(aRootBounds)),
+        mBoundingClientRect(std::move(aBoundingClientRect)),
+        mIntersectionRect(std::move(aIntersectionRect)),
         mIsIntersecting(aIsIntersecting),
         mTarget(aTarget),
         mIntersectionRatio(aIntersectionRatio) {}
@@ -138,6 +138,7 @@ class DOMIntersectionObserver final : public nsISupports,
                                       const Maybe<nsRect>& aRootRect,
                                       const nsRect& aTargetRect,
                                       const Maybe<nsRect>& aIntersectionRect,
+                                      bool aIsIntersecting,
                                       double aIntersectionRatio);
 
   nsCOMPtr<nsPIDOMWindowInner> mOwner;

testing/web-platform/meta/intersection-observer/initial-observation-with-threshold.html.ini

@@ -43,7 +43,8 @@
   assert_equals(entries.length, 2);
   assert_true(entries[1].intersectionRatio >= 0.5 &&
               entries[1].intersectionRatio < 1);
-  assert_equals(entries[1].isIntersecting, true);
+  // See https://github.com/w3c/IntersectionObserver/issues/432
+  assert_equals(entries[1].isIntersecting, false);
   scroller.scrollTop = 100;
 }