Wait for mousemove DOM event, before simulating mousedown for dragstart tests.

chrome/browser/ui/views/drag_and_drop_interactive_uitest.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <algorithm>
 #include <initializer_list>
 #include <memory>
 #include <string>
 #include <vector>
 
@@ skipping 561 matching lines @@
                           const std::string& filename) {
     AssertTestPageIsLoaded();
     return NavigateNamedFrame("right", origin, filename);
   }
 
   ////////////////////////////////////////////////////////////
   // Simulation of starting a drag-and-drop (using the mouse).
 
   bool SimulateMouseDownAndDragStartInLeftFrame() {
     AssertTestPageIsLoaded();
-    if (!SimulateMouseMove(kMiddleOfLeftFrame) || !SimulateMouseDown() ||
-        !SimulateMouseMove(expected_location_of_drag_start_in_left_frame()))
+
+    // Waiting until the mousemove and mousedown events reach the right renderer
+    // is needed to avoid flakiness reported in https://crbug.com/671445 (which
+    // has its root cause in https://crbug.com/647378).  Once the latter bug
+    // is fixed, we should no longer need to wait for these events (because
+    // fixing https://crbug.com/647378 should guarantee that events arrive
+    // to the renderer in the right order).
+    DOMDragEventWaiter mouse_move_event_waiter("mousemove", left_frame());
+    DOMDragEventWaiter mouse_down_event_waiter("mousedown", left_frame());
+
+    if (!SimulateMouseMove(kMiddleOfLeftFrame))
+      return false;
+    if (!mouse_move_event_waiter.WaitForNextMatchingEvent(nullptr))
+      return false;
+
+    if (!SimulateMouseDown())
+      return false;
+    if (!mouse_down_event_waiter.WaitForNextMatchingEvent(nullptr))
+      return false;
+
+    if (!SimulateMouseMove(expected_location_of_drag_start_in_left_frame()))
       return false;
 
     return true;
   }
 
   gfx::Point expected_location_of_drag_start_in_left_frame() {
     // TODO(crbug.com/653490): The delta below should exceed kDragThresholdX and
     // kDragThresholdY from MouseEventManager.cpp in blink.  Ideally, it would
     // come from the OS instead.
     return kMiddleOfLeftFrame + gfx::Vector2d(10, 10);
@@ skipping 545 matching lines @@
                     "dragend"}));
 }
 
 INSTANTIATE_TEST_CASE_P(
     SameSiteSubframe, DragAndDropBrowserTest, ::testing::Values(false));
 
 INSTANTIATE_TEST_CASE_P(
     CrossSiteSubframe, DragAndDropBrowserTest, ::testing::Values(true));
 
 }  // namespace chrome