Bug 1781046 - Add internal ResizeObserver for last remembered size. r=emilio

In a follow-up, it will take care of updating the last remembered size
as described in https://drafts.csswg.org/css-sizing-4/#last-remembered

Differential Revision: https://phabricator.services.mozilla.com/D154325

dom/base/Document.cpp

@@ -2521,6 +2521,7 @@ NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INTERNAL(Document)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mOnloadBlocker)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mLazyLoadImageObserver)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mLazyLoadImageObserverViewport)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mLastRememberedSizeObserver)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mDOMImplementation)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mImageMaps)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mOrientationPendingPromise)
@@ -2640,6 +2641,7 @@ NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(Document)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mDisplayDocument)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mLazyLoadImageObserver)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mLazyLoadImageObserverViewport)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mLastRememberedSizeObserver)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mFontFaceSet)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mReadyForIdle)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mDocumentL10n)
@@ -15789,6 +15791,27 @@ void Document::IncLazyLoadImageReachViewport(bool aLoading) {
   }
 }
 
+ResizeObserver& Document::EnsureLastRememberedSizeObserver() {
+  if (!mLastRememberedSizeObserver) {
+    mLastRememberedSizeObserver =
+        ResizeObserver::CreateLastRememberedSizeObserver(*this);
+  }
+  return *mLastRememberedSizeObserver;
+}
+
+void Document::ObserveForLastRememberedSize(Element& aElement) {
+  // Options are initialized with ResizeObserverBoxOptions::Content_box by
+  // default, which is what we want.
+  static ResizeObserverOptions options;
+  EnsureLastRememberedSizeObserver().Observe(aElement, options);
+}
+
+void Document::UnobserveForLastRememberedSize(Element& aElement) {
+  if (mLastRememberedSizeObserver) {
+    mLastRememberedSizeObserver->Unobserve(aElement);
+  }
+}
+
 void Document::NotifyLayerManagerRecreated() {
   NotifyActivityChanged();
   EnumerateSubDocuments([](Document& aSubDoc) {

dom/base/Document.h

@@ -3748,6 +3748,13 @@ class Document : public nsINode,
   void IncLazyLoadImageStarted() { ++mLazyLoadImageStarted; }
   void IncLazyLoadImageReachViewport(bool aLoading);
 
+  ResizeObserver* GetLastRememberedSizeObserver() {
+    return mLastRememberedSizeObserver;
+  }
+  ResizeObserver& EnsureLastRememberedSizeObserver();
+  void ObserveForLastRememberedSize(Element&);
+  void UnobserveForLastRememberedSize(Element&);
+
   // Dispatch a runnable related to the document.
   nsresult Dispatch(TaskCategory aCategory,
                     already_AddRefed<nsIRunnable>&& aRunnable) final;
@@ -5113,6 +5120,10 @@ class Document : public nsINode,
   // Used to measure how effective the lazyload thresholds are.
   RefPtr<DOMIntersectionObserver> mLazyLoadImageObserverViewport;
 
+  // ResizeObserver for storing and removing the last remembered size.
+  // @see {@link https://drafts.csswg.org/css-sizing-4/#last-remembered}
+  RefPtr<ResizeObserver> mLastRememberedSizeObserver;
+
   // Stack of top layer elements.
   nsTArray<nsWeakPtr> mTopLayer;
 

dom/base/ResizeObserver.cpp

@@ -295,6 +295,18 @@ void ResizeObserver::Observe(Element& aTarget,
   observation =
       new ResizeObservation(aTarget, *this, aOptions.mBox,
                             frame ? frame->GetWritingMode() : WritingMode());
+  if (this == mDocument->GetLastRememberedSizeObserver()) {
+    // Resize observations are initialized with a (0, 0) mLastReportedSize,
+    // this means that the callback won't be called if the element is 0x0.
+    // But we need it called for handling the last remembered size, so set
+    // mLastReportedSize to an invalid size to ensure IsActive() is true
+    // for the current element size.
+    // See https://github.com/w3c/csswg-drafts/issues/3664 about doing this in
+    // the general case, then we won't need this hack for the last remembered
+    // size, and will have consistency with IntersectionObserver.
+    observation->UpdateLastReportedSize(gfx::Size(-1, -1));
+    MOZ_ASSERT(observation->IsActive());
+  }
   mObservationList.insertBack(observation);
 
   // Per the spec, we need to trigger notification in event loop that
@@ -491,6 +503,17 @@ void ResizeObserverEntry::SetDevicePixelContentSize(const gfx::Size& aSize) {
   mDevicePixelContentBoxSize = new ResizeObserverSize(mOwner, aSize, wm);
 }
 
+static void LastRememberedSizeCallback(
+    const Sequence<OwningNonNull<ResizeObserverEntry>>& aEntries,
+    ResizeObserver& aObserver) {
+  // TODO.
+}
+
+/* static */ already_AddRefed<ResizeObserver>
+ResizeObserver::CreateLastRememberedSizeObserver(Document& aDocument) {
+  return do_AddRef(new ResizeObserver(aDocument, LastRememberedSizeCallback));
+}
+
 NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE(ResizeObserverSize, mOwner)
 NS_IMPL_CYCLE_COLLECTING_ADDREF(ResizeObserverSize)
 NS_IMPL_CYCLE_COLLECTING_RELEASE(ResizeObserverSize)

dom/base/ResizeObserver.h

@@ -180,6 +180,9 @@ class ResizeObserver final : public nsISupports, public nsWrapperCache {
    */
   MOZ_CAN_RUN_SCRIPT uint32_t BroadcastActiveObservations();
 
+  static already_AddRefed<ResizeObserver> CreateLastRememberedSizeObserver(
+      Document&);
+
  protected:
   ~ResizeObserver() { Disconnect(); }