Short term workaround to deal with calculation error caused by inverted matrix.

ui/base/events/event.cc

 // Copyright (c) 2012 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/base/events/event.h"
 
 #if defined(USE_X11)
 #include <X11/Xlib.h>
 #endif
 
@@ skipping 256 matching lines @@
       system_location_(0, 0) {
 }
 
 void LocatedEvent::UpdateForRootTransform(
     const gfx::Transform& root_transform) {
   // Transform has to be done at root level.
   gfx::Point3F p(location_);
   root_transform.TransformPointReverse(p);
   // TODO(oshima): Translating a point using reversed matrix can
   // results in small error like 0 -> -0.01, whose floored value
-  // is -1 instead of 0. Investigate the best way to handle this,
-  // instead of just rounding it.
+  // is -1 instead of 0. crbug.com/222483.
   root_location_ = location_ = gfx::ToFlooredPoint(p.AsPointF());
 }
 
 ////////////////////////////////////////////////////////////////////////////////
 // MouseEvent
 
 MouseEvent::MouseEvent(const base::NativeEvent& native_event)
     : LocatedEvent(native_event),
       changed_button_flags_(
           GetChangedMouseButtonFlagsFromNative(native_event)) {
@@ skipping 441 matching lines @@
 int GestureEvent::GetLowestTouchId() const {
   if (touch_ids_bitfield_ == 0)
     return -1;
   int i = -1;
   // Find the index of the least significant 1 bit
   while (!(1 << ++i & touch_ids_bitfield_));
   return i;
 }
 
 }  // namespace ui