Invalid press events don't create multiple pointers with the same id.

ui/events/gestures/motion_event_aura.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.
 
 // MSVC++ requires this to be set before any other includes to get M_PI.
 #define _USE_MATH_DEFINES
 
 #include "ui/events/gestures/motion_event_aura.h"
 
 #include <cmath>
@@ skipping 64 matching lines @@
 }
 
 MotionEventAura::~MotionEventAura() {
 }
 
 bool MotionEventAura::OnTouch(const TouchEvent& touch) {
   int index = FindPointerIndexOfId(touch.touch_id());
   bool pointer_id_is_active = index != -1;
 
   if (touch.type() == ET_TOUCH_PRESSED && pointer_id_is_active) {
-    // Ignore touch press events if we already believe the pointer is down.
+    // TODO(tdresser): This should return false (or NOTREACHED()), and
+    // ignore the touch; however, there is at least one case where we
+    // need to allow a touch press from a currently used touch id. See
+    // crbug.com/446852 for details.
 
-    // TODO(tdresser): this should return false (or NOTREACHED());
-    // however, there is at least one case where we need to allow a
-    // touch press from a currently used touch id. See
-    // crbug.com/446852 for details.
+    // Update the existing touch location. See crbug.com/450880 for an
+    // example of why this is necessary.
+    UpdateTouch(touch);

[jdduke (slow), 2015/01/26 17:16:19]

I guess the alternative is cancelling the previously pressed touch and using the
usual update flow, e.g.

if (touch.type() == ET_TOUCH_PRESSED && pointer_id_is_active) {
  // ...
  OnTouch(TouchEvent(ET_TOUCH_CANCELLED, touch.location(), touch.touch_id(),
touch.time-stamp());
  DCHECK_EQ(-1, FindPointerIndexOfId(touch.touch_id());
} else ...

Not sure if that's any cleaner?

[tdresser, 2015/01/26 17:48:03]

Done.

+    UpdateCachedAction(touch);
+    set_flags(touch.flags());
+    set_event_time(touch.time_stamp() + base::TimeTicks());
+    return true;
   } else if (touch.type() != ET_TOUCH_PRESSED && !pointer_id_is_active) {
     // We could have an active touch stream transfered to us, resulting in touch
     // move or touch up events without associated touch down events. Ignore
     // them.
     return false;
   }
 
   if (touch.type() == ET_TOUCH_MOVED && touch.x() == GetX(index) &&
       touch.y() == GetY(index)) {
     return false;
@@ skipping 90 matching lines @@
 }
 
 int MotionEventAura::GetIndexFromId(int id) const {
   int index = FindPointerIndexOfId(id);
   DCHECK_GE(index, 0);
   DCHECK_LT(index, static_cast<int>(GetPointerCount()));
   return index;
 }
 
 }  // namespace ui