Bug 1617154 - [intersection-observer] Accept a Document as an explicit root. r=emilio,bzbarsky

This patch allows users to specify a Document as an explicit root of an
intersection observer. For rationale, see the intent-to thread:
https://groups.google.com/forum/#!topic/mozilla.dev.platform/64nDLTAZGzY

It is implemented under a preference option enabled in Nightly:
dom.IntersectionObserverExplicitDocumentRoot.enabled
When disabled, the current TypeError exception is preserved so that
people can continue to feature detect the support.

The enhancement is tested by and enabled for the following test:
https://w3c-test.org/intersection-observer/document-scrolling-element-root.html

Differential Revision: https://phabricator.services.mozilla.com/D63766

--HG--
extra : moz-landing-system : lando

dom/base/DOMIntersectionObserver.cpp

@@ -12,6 +12,7 @@
 #include "mozilla/PresShell.h"
 #include "mozilla/StaticPrefs_dom.h"
 #include "mozilla/ServoBindings.h"
+#include "mozilla/StaticPrefs_dom.h"
 #include "mozilla/dom/BrowserChild.h"
 #include "mozilla/dom/BrowsingContext.h"
 #include "mozilla/dom/DocumentInlines.h"
@@ -91,7 +92,20 @@ already_AddRefed<DOMIntersectionObserver> DOMIntersectionObserver::Constructor(
   RefPtr<DOMIntersectionObserver> observer =
       new DOMIntersectionObserver(window.forget(), aCb);
 
-  observer->mRoot = aOptions.mRoot;
+  if (!aOptions.mRoot.IsNull()) {
+    if (aOptions.mRoot.Value().IsElement()) {
+      observer->mRoot = aOptions.mRoot.Value().GetAsElement();
+    } else {
+      MOZ_ASSERT(aOptions.mRoot.Value().IsDocument());
+      if (!StaticPrefs::
+              dom_IntersectionObserverExplicitDocumentRoot_enabled()) {
+        aRv.ThrowTypeError<dom::MSG_DOES_NOT_IMPLEMENT_INTERFACE>(
+            u"'root' member of IntersectionObserverInit", u"Element");
+        return nullptr;
+      }
+      observer->mRoot = aOptions.mRoot.Value().GetAsDocument();
+    }
+  }
 
   if (!observer->SetRootMargin(aOptions.mRootMargin)) {
     aRv.ThrowSyntaxError("rootMargin must be specified in pixels or percent.");
@@ -257,7 +271,7 @@ enum class BrowsingContextOrigin { Similar, Different, Unknown };
 // contexts" is gone, but this is still in the spec, see
 // https://github.com/w3c/IntersectionObserver/issues/161
 static BrowsingContextOrigin SimilarOrigin(const Element& aTarget,
-                                           const Element* aRoot) {
+                                           const nsINode* aRoot) {
   if (!aRoot) {
     return BrowsingContextOrigin::Unknown;
   }
@@ -466,10 +480,10 @@ void DOMIntersectionObserver::Update(Document* aDocument,
   // document.
   nsRect rootRect;
   nsIFrame* rootFrame = nullptr;
-  Element* root = mRoot;
+  nsINode* root = mRoot;
   Maybe<nsRect> remoteDocumentVisibleRect;
-  if (mRoot) {
-    if ((rootFrame = mRoot->GetPrimaryFrame())) {
+  if (mRoot && mRoot->IsElement()) {
+    if ((rootFrame = mRoot->AsElement()->GetPrimaryFrame())) {
       nsRect rootRectRelativeToRootFrame;
       if (rootFrame->IsScrollFrame()) {
         // rootRectRelativeToRootFrame should be the content rect of rootFrame,
@@ -485,21 +499,26 @@ void DOMIntersectionObserver::Update(Document* aDocument,
       rootRect = nsLayoutUtils::TransformFrameRectToAncestor(
           rootFrame, rootRectRelativeToRootFrame, containingBlock);
     }
-  } else if (Document* topLevelDocument = GetTopLevelDocument(*aDocument)) {
-    if (PresShell* presShell = topLevelDocument->GetPresShell()) {
-      rootFrame = presShell->GetRootScrollFrame();
-      if (rootFrame) {
-        root = rootFrame->GetContent()->AsElement();
-        nsIScrollableFrame* scrollFrame = do_QueryFrame(rootFrame);
-        rootRect = scrollFrame->GetScrollPortRect();
+  } else {
+    MOZ_ASSERT(!mRoot || mRoot->IsDocument());
+    Document* rootDocument =
+        mRoot ? mRoot->AsDocument() : GetTopLevelDocument(*aDocument);
+    if (rootDocument) {
+      if (PresShell* presShell = rootDocument->GetPresShell()) {
+        rootFrame = presShell->GetRootScrollFrame();
+        if (rootFrame) {
+          root = rootFrame->GetContent()->AsElement();
+          nsIScrollableFrame* scrollFrame = do_QueryFrame(rootFrame);
+          rootRect = scrollFrame->GetScrollPortRect();
+        }
       }
+    } else if (Maybe<OopIframeMetrics> metrics =
+                   GetOopIframeMetrics(*aDocument)) {
+      // `implicit root` case in an out-of-process iframe.
+      rootFrame = metrics->mInProcessRootFrame;
+      rootRect = metrics->mInProcessRootRect;
+      remoteDocumentVisibleRect = Some(metrics->mRemoteDocumentVisibleRect);
     }
-  } else if (Maybe<OopIframeMetrics> metrics =
-                 GetOopIframeMetrics(*aDocument)) {
-    // `implicit root` case in an out-of-process iframe.
-    rootFrame = metrics->mInProcessRootFrame;
-    rootRect = metrics->mInProcessRootRect;
-    remoteDocumentVisibleRect = Some(metrics->mRemoteDocumentVisibleRect);
   }
 
   nsMargin rootMargin;  // This root margin is NOT applied in `implicit root`

dom/base/DOMIntersectionObserver.h

@@ -118,7 +118,7 @@ class DOMIntersectionObserver final : public nsISupports,
 
   nsISupports* GetParentObject() const { return mOwner; }
 
-  Element* GetRoot() const { return mRoot; }
+  nsINode* GetRoot() const { return mRoot; }
 
   void GetRootMargin(DOMString& aRetVal);
   void GetThresholds(nsTArray<double>& aRetVal);
@@ -151,7 +151,7 @@ class DOMIntersectionObserver final : public nsISupports,
   RefPtr<Document> mDocument;
   Variant<RefPtr<dom::IntersectionCallback>, NativeIntersectionObserverCallback>
       mCallback;
-  RefPtr<Element> mRoot;
+  RefPtr<nsINode> mRoot;
   StyleRect<LengthPercentage> mRootMargin;
   nsTArray<double> mThresholds;
 

dom/webidl/IntersectionObserver.webidl

@@ -34,7 +34,7 @@ interface IntersectionObserver {
               optional IntersectionObserverInit options = {});
 
   [Constant]
-  readonly attribute Element? root;
+  readonly attribute Node? root;
   [Constant]
   readonly attribute DOMString rootMargin;
   [Constant,Cached]
@@ -57,7 +57,7 @@ dictionary IntersectionObserverEntryInit {
 };
 
 dictionary IntersectionObserverInit {
-  Element?  root = null;
+  (Element or Document)?  root = null;
   DOMString rootMargin = "0px";
   (double or sequence<double>) threshold = 0;
 };

modules/libpref/init/StaticPrefList.yaml

@@ -1749,6 +1749,11 @@
   value: true
   mirror: always
 
+- name: dom.IntersectionObserverExplicitDocumentRoot.enabled
+  type: bool
+  value: @IS_NIGHTLY_BUILD@
+  mirror: always
+
 - name: dom.ipc.cancel_content_js_when_navigating
   type: bool
   value: true

testing/web-platform/meta/intersection-observer/__dir__.ini

@@ -1 +1 @@
-prefs: [dom.IntersectionObserver.enabled:true]
+prefs: [dom.IntersectionObserver.enabled:true, dom.IntersectionObserverExplicitDocumentRoot.enabled:true]

testing/web-platform/meta/intersection-observer/document-scrolling-element-root.html.ini

@@ -1,4 +0,0 @@
-[document-scrolling-element-root.html]
-  [Observer with explicit root which is the document.]
-    expected: FAIL
-