Bug 1815140 Part 1: Make Grid dom object an idempotent property of its frame. r=emilio

This ensures that repeated calls to Element::GetGridFragments will return
an array of idempotent Grid objects for each fragment. This is
accomplished by making the Grid object hold a WeakFrame back to its
originating frame, and updating a property on construction and
destruction.

Differential Revision: https://phabricator.services.mozilla.com/D169724

dom/base/Element.cpp

@@ -3563,7 +3563,16 @@ void Element::GetGridFragments(nsTArray<RefPtr<Grid>>& aResult) {
   // If we get a nsGridContainerFrame from the prior call,
   // all the next-in-flow frames will also be nsGridContainerFrames.
   while (frame) {
-    aResult.AppendElement(new Grid(this, frame));
+    // Get the existing Grid object, if it exists. This object is
+    // guaranteed to be up-to-date because GetGridFrameWithComputedInfo
+    // will delete an existing one when regenerating grid info.
+    Grid* gridFragment = frame->GetGridFragmentInfo();
+    if (!gridFragment) {
+      // Grid constructor will add itself as a property to frame, and the
+      // destructor will remove itself if the frame still exists.
+      gridFragment = new Grid(this, frame);
+    }
+    aResult.AppendElement(gridFragment);
     frame = static_cast<nsGridContainerFrame*>(frame->GetNextInFlow());
   }
 }

dom/grid/Grid.cpp

@@ -25,11 +25,14 @@ NS_INTERFACE_MAP_END
 
 Grid::Grid(nsISupports* aParent, nsGridContainerFrame* aFrame)
     : mParent(do_QueryInterface(aParent)),
+      mFrame(aFrame),
       mRows(new GridDimension(this)),
       mCols(new GridDimension(this)) {
   MOZ_ASSERT(aFrame,
              "Should never be instantiated with a null nsGridContainerFrame");
 
+  aFrame->SetProperty(nsGridContainerFrame::GridFragmentInfo(), this);
+
   // Construct areas first, because lines may need to reference them
   // to extract additional names for boundary lines.
 
@@ -80,7 +83,14 @@ Grid::Grid(nsISupports* aParent, nsGridContainerFrame* aFrame)
   mCols->SetLineInfo(columnTrackInfo, columnLineInfo, mAreas, false);
 }
 
-Grid::~Grid() = default;
+Grid::~Grid() {
+  // If mFrame is still alive, clear the property that points back to us.
+  if (mFrame.IsAlive()) {
+    mFrame->RemoveProperty(nsGridContainerFrame::GridFragmentInfo());
+  }
+
+  // Our member variables are correctly handled by default destruction.
+}
 
 JSObject* Grid::WrapObject(JSContext* aCx, JS::Handle<JSObject*> aGivenProto) {
   return Grid_Binding::Wrap(aCx, this, aGivenProto);

dom/grid/Grid.h

@@ -38,6 +38,7 @@ class Grid : public nsISupports, public nsWrapperCache {
 
  protected:
   nsCOMPtr<Element> mParent;
+  WeakFrame mFrame;
   RefPtr<GridDimension> mRows;
   RefPtr<GridDimension> mCols;
   nsTArray<RefPtr<GridArea>> mAreas;

layout/generic/nsGridContainerFrame.cpp

@@ -8919,6 +8919,11 @@ void nsGridContainerFrame::Reflow(nsPresContext* aPresContext,
     // since this bit is only set by accessing a ChromeOnly property, and
     // therefore can't unduly slow down normal web browsing.
 
+    // Clear our GridFragmentInfo property, which might be holding a stale
+    // dom::Grid object built from previously-computed info. This will
+    // ensure that the next call to GetGridFragments will create a new one.
+    RemoveProperty(GridFragmentInfo());
+
     // Now that we know column and row sizes and positions, set
     // the ComputedGridTrackInfo and related properties
 

layout/generic/nsGridContainerFrame.h

@@ -18,6 +18,9 @@
 
 namespace mozilla {
 class PresShell;
+namespace dom {
+class Grid;
+}
 }  // namespace mozilla
 
 /**
@@ -214,6 +217,18 @@ class nsGridContainerFrame final : public nsContainerFrame,
     return info;
   }
 
+  /**
+   * This property is set by the creation of a dom::Grid object, and cleared by
+   * its destructor. Since the Grid object manages the lifecycle, the property
+   * itself is set without a destructor. The property is also cleared whenever
+   * new grid computed info is generated during reflow, ensuring that we aren't
+   * holding a stale dom::Grid object.
+   */
+  NS_DECLARE_FRAME_PROPERTY_WITHOUT_DTOR(GridFragmentInfo, mozilla::dom::Grid)
+  mozilla::dom::Grid* GetGridFragmentInfo() {
+    return GetProperty(GridFragmentInfo());
+  }
+
   struct AtomKey {
     RefPtr<nsAtom> mKey;