diff --git a/gfx/layers/ipc/CompositorBridgeParent.cpp b/gfx/layers/ipc/CompositorBridgeParent.cpp
index a175e6c51846..aa41317baa7f 100644
--- a/gfx/layers/ipc/CompositorBridgeParent.cpp
+++ b/gfx/layers/ipc/CompositorBridgeParent.cpp
@@ -259,6 +259,9 @@ void CompositorBridgeParent::InitSameProcess(widget::CompositorWidget* aWidget,
 
   mWidget = aWidget;
   mRootLayerTreeID = aLayerTreeId;
+#if defined(XP_WIN)
+  mWidget->AsWindows()->SetRootLayerTreeID(mRootLayerTreeID);
+#endif
 
   Initialize();
 }
diff --git a/widget/windows/InProcessWinCompositorWidget.cpp b/widget/windows/InProcessWinCompositorWidget.cpp
index 21cec6237885..89729f7f0a59 100644
--- a/widget/windows/InProcessWinCompositorWidget.cpp
+++ b/widget/windows/InProcessWinCompositorWidget.cpp
@@ -360,4 +360,15 @@ void InProcessWinCompositorWidget::ObserveVsync(VsyncObserver* aObserver) {
   }
 }
 
+void InProcessWinCompositorWidget::UpdateCompositorWnd(
+    const HWND aCompositorWnd, const HWND aParentWnd) {
+  MOZ_ASSERT(layers::CompositorThreadHolder::IsInCompositorThread());
+  MOZ_ASSERT(aCompositorWnd && aParentWnd);
+  MOZ_ASSERT(aParentWnd == mWnd);
+
+  // Since we're in the parent process anyway, we can just call SetParent
+  // directly.
+  ::SetParent(aCompositorWnd, aParentWnd);
+  mSetParentCompleted = true;
+}
 }  // namespace mozilla::widget
diff --git a/widget/windows/InProcessWinCompositorWidget.h b/widget/windows/InProcessWinCompositorWidget.h
index 783678eb94ff..5c0d34e13f6f 100644
--- a/widget/windows/InProcessWinCompositorWidget.h
+++ b/widget/windows/InProcessWinCompositorWidget.h
@@ -71,7 +71,7 @@ class InProcessWinCompositorWidget final
   nsIWidget* RealWidget() override;
 
   void UpdateCompositorWnd(const HWND aCompositorWnd,
-                           const HWND aParentWnd) override {}
+                           const HWND aParentWnd) override;
   void SetRootLayerTreeID(const layers::LayersId& aRootLayerTreeId) override {}
 
  private:
diff --git a/widget/windows/WinCompositorWidget.cpp b/widget/windows/WinCompositorWidget.cpp
index 3f0dc92a5b5f..15957a1c3f59 100644
--- a/widget/windows/WinCompositorWidget.cpp
+++ b/widget/windows/WinCompositorWidget.cpp
@@ -70,10 +70,10 @@ void WinCompositorWidget::UpdateCompositorWndSizeIfNecessary() {
     return;
   }
 
-  // This code is racing with the compositor, which needs to reparent
-  // the compositor surface to the actual window (mWnd). To avoid racing
-  // mutations, we refuse to proceed until ::SetParent() is called in parent
-  // process. After the ::SetParent() call, composition is scheduled in
+  // This code is racing with the compositor, which needs to reparent the
+  // compositor surface to the actual window (mWnd). To avoid racing mutations,
+  // we refuse to proceed until ::SetParent() is called in the parent process.
+  // After the ::SetParent() call, composition is scheduled in
   // CompositorWidgetParent::UpdateCompositorWnd().
   if (!mSetParentCompleted) {
     // ::SetParent() is not completed yet.