[IntersectionObserver] Fix handling of remote parent frame intersection

If a process-isolated iframe has a zero-area intersection with its
parent frame, implicit root observations should not be able to have
isIntersecting=true.

BUG=1070332
R=vmpstr@chromium.org

Change-Id: I05e8a05af14e84d493609bbc1675a28793dc1823
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2166605
Commit-Queue: vmpstr <vmpstr@chromium.org>
Reviewed-by: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#762875}

third_party/blink/renderer/core/intersection_observer/intersection_geometry.cc

@@ -473,11 +473,16 @@ bool IntersectionGeometry::ClipToRoot(const LayoutObject* root,
       // done yet.
       LocalFrame* local_root_frame = root->GetDocument().GetFrame();
       IntRect clip_rect(local_root_frame->RemoteViewportIntersection());
-      // Map clip_rect from the coordinate system of the local root frame to
-      // the coordinate system of the remote main frame.
-      clip_rect.MoveBy(IntPoint(local_root_frame->RemoteViewportOffset()));
-      does_intersect &=
-          intersection_rect.InclusiveIntersect(PhysicalRect(clip_rect));
+      if (clip_rect.IsEmpty()) {
+        intersection_rect = PhysicalRect();
+        does_intersect = false;
+      } else {
+        // Map clip_rect from the coordinate system of the local root frame to
+        // the coordinate system of the remote main frame.
+        clip_rect.MoveBy(IntPoint(local_root_frame->RemoteViewportOffset()));
+        does_intersect &=
+            intersection_rect.InclusiveIntersect(PhysicalRect(clip_rect));
+      }
     }
   }
 

third_party/blink/web_tests/external/wpt/intersection-observer/resources/cross-origin-subframe.html

@@ -29,10 +29,15 @@ function entryToJson(entry) {
     intersectionRect: clientRectToJson(entry.intersectionRect),
     rootBounds: clientRectToJson(entry.rootBounds),
     isIntersecting: entry.isIntersecting,
-    target: entry.target.id
+    target: entry.target === document.documentElement ? "html" : entry.target.id
   };
 }
 
+function boundingClientRectToJson(element) {
+  let r = element.getBoundingClientRect();
+  return [r.left, r.right, r.top, r.bottom];
+}
+
 // Note that we never use RAF in this code, because this frame might get render-throttled.
 // Instead of RAF-ing, we just post an empty message to the parent window, which will
 // RAF when it is received, and then send us a message to cause the next step to run.
@@ -43,6 +48,7 @@ var observer = new IntersectionObserver(function(changes) {
 }, { rootMargin: "7px" });
 observer.observe(target);
 observer.observe(emptyTarget);
+observer.observe(document.documentElement);
 
 function step0() {
   entries = entries.concat(observer.takeRecords());
@@ -59,6 +65,12 @@ function step0() {
     rootBounds: "null",
     isIntersecting: false,
     target: emptyTarget.id
+  }, {
+    boundingClientRect: boundingClientRectToJson(document.documentElement),
+    intersectionRect: [0, 0, 0, 0],
+    rootBounds: "null",
+    isIntersecting: false,
+    target: "html"
   }];
   port.postMessage({
     actual: entries.map(entryToJson),
@@ -71,9 +83,21 @@ function step0() {
 
 function step1() {
   entries = entries.concat(observer.takeRecords());
+  var client_rect = boundingClientRectToJson(document.documentElement);
+  // When the top document is scrolled all the way up, the iframe element is
+  // 108px below the scrolling viewport, and the iframe has a 2px border. When
+  // the top document is scrolled to y=200, the top 90px of the iframe's content
+  // is visible.
+  var expected = [{
+    boundingClientRect: client_rect,
+    intersectionRect: client_rect.slice(0, 3).concat(90),
+    rootBounds: "null",
+    isIntersecting: true,
+    target: "html"
+  }];
   port.postMessage({
     actual: entries.map(entryToJson),
-    expected: [],
+    expected: expected,
     description: "topDocument.scrollingElement.scrollTop = 200"
   }, "*");
   entries = [];
@@ -121,6 +145,12 @@ function step3() {
     rootBounds: "null",
     isIntersecting: false,
     target: emptyTarget.id
+  }, {
+    boundingClientRect: boundingClientRectToJson(document.documentElement),
+    intersectionRect: [0, 0, 0, 0],
+    rootBounds: "null",
+    isIntersecting: false,
+    target: "html"
   }];
   port.postMessage({
     actual: entries.map(entryToJson),