aura: Turn XInput2 back on.

ui/base/x/events_x.cc

Index: ui/base/x/events_x.cc
diff --git a/ui/base/x/events_x.cc b/ui/base/x/events_x.cc
index 06adb00c37d2620caf5889573d236d70dd6e08fa..355fa3fab4b9dd90e43cb4e09878269dd5e6c005 100644
--- a/ui/base/x/events_x.cc
+++ b/ui/base/x/events_x.cc
@@ -256,8 +256,31 @@ gfx::Point EventLocationFromNative(const base::NativeEvent& native_event) {
     case GenericEvent: {
       XIDeviceEvent* xievent =
           static_cast<XIDeviceEvent*>(native_event->xcookie.data);
-      return gfx::Point(static_cast<int>(xievent->event_x),
-                        static_cast<int>(xievent->event_y));
+
+      if (xievent->sourceid == xievent->deviceid) {
+        // This event is coming from a slave device. Read the position from the
+        // valuators, because the events reported for a slave device seems to be
+        // behind the master device by one event. See more on crbug.com/103981.
+        // The position in the valuators is in the global screen coordinates.
+        // But it is necessary to convert it into the window's coordinates. If
+        // the valuator is not set, that means the value hasn't changed, and so
+        // we can use the value from event_x/event_y (which are in the window's
+        // coordinates).
+        double* valuators = xievent->valuators.values;
+
+        double x = xievent->event_x;
+        if (XIMaskIsSet(xievent->valuators.mask, 0))
+          x = *valuators++ - (xievent->root_x - xievent->event_x);
+
+        double y = xievent->event_y;
+        if (XIMaskIsSet(xievent->valuators.mask, 1))
+          y = *valuators++ - (xievent->root_y - xievent->event_y);
+
+        return gfx::Point(static_cast<int>(x), static_cast<int>(y));
+      } else {
+        return gfx::Point(static_cast<int>(xievent->event_x),
+                          static_cast<int>(xievent->event_y));
+      }
     }
   }
   return gfx::Point();