--- a/gfx/ipc/GPUParent.cpp
+++ b/gfx/ipc/GPUParent.cpp
@@ -229,19 +229,19 @@ GPUParent::RecvInitImageBridge(Endpoint<
 mozilla::ipc::IPCResult
 GPUParent::RecvInitVRManager(Endpoint<PVRManagerParent>&& aEndpoint)
 {
   VRManagerParent::CreateForGPUProcess(Move(aEndpoint));
   return IPC_OK();
 }
 
 mozilla::ipc::IPCResult
-GPUParent::RecvInitUiCompositorController(Endpoint<PUiCompositorControllerParent>&& aEndpoint)
+GPUParent::RecvInitUiCompositorController(const uint64_t& aRootLayerTreeId, Endpoint<PUiCompositorControllerParent>&& aEndpoint)
 {
-  UiCompositorControllerParent::Start(Move(aEndpoint));
+  UiCompositorControllerParent::Start(aRootLayerTreeId, Move(aEndpoint));
   return IPC_OK();
 }
 
 mozilla::ipc::IPCResult
 GPUParent::RecvUpdatePref(const GfxPrefSetting& setting)
 {
   gfxPrefs::Pref* pref = gfxPrefs::all()[setting.index()];
   pref->SetCachedValue(setting.value());

--- a/gfx/ipc/GPUParent.h
+++ b/gfx/ipc/GPUParent.h
@@ -32,17 +32,17 @@ public:
 
   mozilla::ipc::IPCResult RecvInit(nsTArray<GfxPrefSetting>&& prefs,
                                    nsTArray<GfxVarUpdate>&& vars,
                                    const DevicePrefs& devicePrefs,
                                    nsTArray<LayerTreeIdMapping>&& mappings) override;
   mozilla::ipc::IPCResult RecvInitVsyncBridge(Endpoint<PVsyncBridgeParent>&& aVsyncEndpoint) override;
   mozilla::ipc::IPCResult RecvInitImageBridge(Endpoint<PImageBridgeParent>&& aEndpoint) override;
   mozilla::ipc::IPCResult RecvInitVRManager(Endpoint<PVRManagerParent>&& aEndpoint) override;
-  mozilla::ipc::IPCResult RecvInitUiCompositorController(Endpoint<PUiCompositorControllerParent>&& aEndpoint) override;
+  mozilla::ipc::IPCResult RecvInitUiCompositorController(const uint64_t& aRootLayerTreeId, Endpoint<PUiCompositorControllerParent>&& aEndpoint) override;
   mozilla::ipc::IPCResult RecvUpdatePref(const GfxPrefSetting& pref) override;
   mozilla::ipc::IPCResult RecvUpdateVar(const GfxVarUpdate& pref) override;
   mozilla::ipc::IPCResult RecvNewWidgetCompositor(
       Endpoint<PCompositorBridgeParent>&& aEndpoint,
       const CSSToLayoutDeviceScale& aScale,
       const TimeDuration& aVsyncRate,
       const CompositorOptions& aOptions,
       const bool& aUseExternalSurface,

--- a/gfx/ipc/PGPU.ipdl
+++ b/gfx/ipc/PGPU.ipdl
@@ -53,17 +53,17 @@ parent:
   async Init(GfxPrefSetting[] prefs,
              GfxVarUpdate[] vars,
              DevicePrefs devicePrefs,
              LayerTreeIdMapping[] mapping);
 
   async InitVsyncBridge(Endpoint<PVsyncBridgeParent> endpoint);
   async InitImageBridge(Endpoint<PImageBridgeParent> endpoint);
   async InitVRManager(Endpoint<PVRManagerParent> endpoint);
-  async InitUiCompositorController(Endpoint<PUiCompositorControllerParent> endpoint);
+  async InitUiCompositorController(uint64_t rootLayerTreeId, Endpoint<PUiCompositorControllerParent> endpoint);
 
   // Called to update a gfx preference or variable.
   async UpdatePref(GfxPrefSetting pref);
   async UpdateVar(GfxVarUpdate var);
 
   // Create a new top-level compositor.
   async NewWidgetCompositor(Endpoint<PCompositorBridgeParent> endpoint,
                             CSSToLayoutDeviceScale scale,