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

ui/events/gestures/motion_event_aura.cc

Index: ui/events/gestures/motion_event_aura.cc
diff --git a/ui/events/gestures/motion_event_aura.cc b/ui/events/gestures/motion_event_aura.cc
index 2f532e7f325f6ad0e9f5d211bdba2707ca89834f..a2ac480a658c7b4a37ef66171f97199df58ccec6 100644
--- a/ui/events/gestures/motion_event_aura.cc
+++ b/ui/events/gestures/motion_event_aura.cc
@@ -82,12 +82,18 @@ bool MotionEventAura::OnTouch(const TouchEvent& touch) {
   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());
-    // however, there is at least one case where we need to allow a
-    // touch press from a currently used touch id. See
+    // 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.
+
+    // 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