The helper functions for slop region check and subtraction are modified.

ui/events/gesture_detection/gesture_provider_unittest.cc

 // Copyright 2014 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 "ui/events/gesture_detection/gesture_provider.h"
 
 #include <stddef.h>
 
 #include <memory>
 
@@ skipping 1046 matching lines @@
       event_time, MotionEvent::ACTION_MOVE, 0, scaled_touch_slop / 2,
       kMaxTwoFingerTapSeparation / 2, 0, 2 * kMaxTwoFingerTapSeparation, 0);
   EXPECT_TRUE(gesture_provider_->OnTouchEvent(event));
 
   EXPECT_EQ(ET_GESTURE_TAP_DOWN, GetReceivedGesture(0).type());
   EXPECT_EQ(ET_GESTURE_SCROLL_BEGIN, GetReceivedGesture(1).type());
   EXPECT_EQ(ET_GESTURE_SCROLL_UPDATE, GetReceivedGesture(2).type());
   EXPECT_EQ(3U, GetReceivedGestureCount());
 }
 
+TEST_F(GestureProviderTest, ScrollStartWithSecondaryPointer) {

[tdresser, 2016/07/22 15:57:45]

Out of curiosity, what ended up being the solution to your previous problem
here?

[sahel, 2016/07/22 19:46:42]

Creating events with the sequence of action_down, action_pointer_down,
action_pointer_up (even with setting the action index to be the first pointer),
didn't help.

Because each time new events were created and the ids would be set
appropriately. 

I used utility functions on the same event instead.
I also had to add a new utility function that can release a pointer with a given
index rather than always the last pointer.

+  EnableTwoFingerTap(kMaxTwoFingerTapSeparation, base::TimeDelta());
+  const float scaled_touch_slop = GetTouchSlop();
+
+  base::TimeTicks event_time = base::TimeTicks::Now();
+
+  MockMotionEvent event =
+      ObtainMotionEvent(event_time, MotionEvent::ACTION_DOWN, 0, 0);
+  EXPECT_TRUE(gesture_provider_->OnTouchEvent(event));
+
+  // Wait for the two finger tap timeout, and put the second pointer down.
+  event_time += kOneSecond;
+  event = ObtainMotionEvent(event_time, MotionEvent::ACTION_POINTER_DOWN, 0, 0,
+                            kMaxTwoFingerTapSeparation / 2, 0);
+  EXPECT_TRUE(gesture_provider_->OnTouchEvent(event));
+
+  event.ReleasePointAt(0);
+  EXPECT_TRUE(gesture_provider_->OnTouchEvent(event));
+
+  event.set_action(MotionEvent::ACTION_MOVE);
+  event.MovePoint(0, kMaxTwoFingerTapSeparation / 2, 2 * scaled_touch_slop);
+  EXPECT_TRUE(gesture_provider_->OnTouchEvent(event));
+
+  event.ReleasePoint();
+  EXPECT_TRUE(gesture_provider_->OnTouchEvent(event));
+
+  EXPECT_EQ(ET_GESTURE_TAP_DOWN, GetReceivedGesture(0).type());
+  EXPECT_EQ(ET_GESTURE_SCROLL_BEGIN, GetReceivedGesture(1).type());
+  EXPECT_EQ(ET_GESTURE_SCROLL_UPDATE, GetReceivedGesture(2).type());
+  EXPECT_EQ(ET_GESTURE_SCROLL_END, GetReceivedGesture(3).type());
+  EXPECT_EQ(4U, GetReceivedGestureCount());
+}
+
 TEST_F(GestureProviderTest, LongPressAndTapCancelledWhenScrollBegins) {
   base::TimeTicks event_time = base::TimeTicks::Now();
 
   MockMotionEvent event =
       ObtainMotionEvent(event_time, MotionEvent::ACTION_DOWN);
   EXPECT_TRUE(gesture_provider_->OnTouchEvent(event));
   event = ObtainMotionEvent(event_time + kOneMicrosecond,
                             MotionEvent::ACTION_MOVE,
                             kFakeCoordX * 5,
                             kFakeCoordY * 5);
@@ skipping 1683 matching lines @@
   event = ObtainMotionEvent(event_time, MotionEvent::ACTION_DOWN, kFakeCoordX,
                             kFakeCoordY + GetTouchSlop() / 2);
   EXPECT_TRUE(gesture_provider_->OnTouchEvent(event));
   event = ObtainMotionEvent(event_time, MotionEvent::ACTION_UP);
   EXPECT_TRUE(gesture_provider_->OnTouchEvent(event));
   EXPECT_EQ(ET_GESTURE_TAP, GetMostRecentGestureEventType());
   EXPECT_EQ(1, GetMostRecentGestureEvent().details.tap_count());
 }
 
 }  // namespace ui