[widget] Change RegisterDragDrop to be called on idle.

This is a low-priority init function that should not be called with
immediate dispatch to the current thread, for performance reasons.
Additionally, do not call RegisterDragDrop for hidden windows.

view/nsView.cpp

@@ -695,7 +695,10 @@ nsresult nsView::AttachToTopLevelWidget(nsIWidget* aWidget)
   mWindow = aWidget;
 
   mWindow->SetAttachedWidgetListener(this);
-  mWindow->EnableDragDrop(true);
+  if (mWindow->WindowType() != eWindowType_invisible) {
+    nsresult rv = mWindow->AsyncEnableDragDrop(true);
+    NS_ENSURE_SUCCESS(rv, rv);
+  }
   mWidgetIsTopLevel = true;
 
   // Refresh the view bounds

widget/nsBaseWidget.cpp

@@ -2133,6 +2133,16 @@ nsBaseWidget::UpdateSynthesizedTouchState(MultiTouchInput* aState,
   return inputToDispatch;
 }
 
+nsresult
+nsBaseWidget::AsyncEnableDragDrop(bool aEnable)
+{
+  RefPtr<nsBaseWidget> kungFuDeathGrip = this;
+  return NS_IdleDispatchToCurrentThread(
+    NS_NewRunnableFunction([this, aEnable, kungFuDeathGrip]() {
+                             EnableDragDrop(aEnable);
+                           }));
+}
+
 void
 nsBaseWidget::RegisterPluginWindowForRemoteUpdates()
 {

widget/nsBaseWidget.h

@@ -243,6 +243,7 @@ public:
   NS_IMETHOD              SetNonClientMargins(LayoutDeviceIntMargin& aMargins) override;
   virtual LayoutDeviceIntPoint GetClientOffset() override;
   virtual void            EnableDragDrop(bool aEnable) override {};
+  virtual nsresult        AsyncEnableDragDrop(bool aEnable) override;
   NS_IMETHOD              GetAttention(int32_t aCycleCount) override;
   virtual bool            HasPendingInputEvent() override;
   NS_IMETHOD              SetIcon(const nsAString &anIconSpec) override;

widget/nsIWidget.h

@@ -1378,6 +1378,7 @@ class nsIWidget : public nsISupports
      * Enables the dropping of files to a widget.
      */
     virtual void EnableDragDrop(bool aEnable) = 0;
+    virtual nsresult AsyncEnableDragDrop(bool aEnable) = 0;
 
     /**
      * Enables/Disables system mouse capture.

widget/windows/nsWindow.cpp

@@ -3734,22 +3734,21 @@ nsWindow::ClientToWindowSize(const LayoutDeviceIntSize& aClientSize)
 void
 nsWindow::EnableDragDrop(bool aEnable)
 {
-  NS_ASSERTION(mWnd, "nsWindow::EnableDragDrop() called after Destroy()");
+  if (!mWnd) {
+    // Return early if the window already closed
+    return;
+  }
 
   nsresult rv = NS_ERROR_FAILURE;
   if (aEnable) {
     if (!mNativeDragTarget) {
       mNativeDragTarget = new nsNativeDragTarget(this);
       mNativeDragTarget->AddRef();
-      if (SUCCEEDED(::CoLockObjectExternal((LPUNKNOWN)mNativeDragTarget,
-                                           TRUE, FALSE))) {
-        ::RegisterDragDrop(mWnd, (LPDROPTARGET)mNativeDragTarget);
-      }
+      ::RegisterDragDrop(mWnd, (LPDROPTARGET)mNativeDragTarget);
     }
   } else {
     if (mWnd && mNativeDragTarget) {
       ::RevokeDragDrop(mWnd);
-      ::CoLockObjectExternal((LPUNKNOWN)mNativeDragTarget, FALSE, TRUE);
       mNativeDragTarget->DragCancel();
       NS_RELEASE(mNativeDragTarget);
     }