Fixes rounding error when calculating MOVE event

components/exo/pointer.cc

 // Copyright 2015 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 "components/exo/pointer.h"
 
 #include "ash/public/cpp/shell_window_ids.h"
 #include "cc/output/copy_output_request.h"
 #include "cc/output/copy_output_result.h"
 #include "components/exo/pointer_delegate.h"
@@ skipping 23 matching lines @@
 
 const float kLargeCursorScale = 2.8f;
 
 // Synthesized events typically lack floating point precision so to avoid
 // generating mouse event jitter we consider the location of these events
 // to be the same as |location| if floored values match.
 bool SameLocation(const ui::LocatedEvent* event, const gfx::PointF& location) {
   if (event->flags() & ui::EF_IS_SYNTHESIZED)
     return event->location() == gfx::ToFlooredPoint(location);
 
-  return event->location_f() == location;
+  // In general, it is good practice to compare floats using an Epsilon.
+  // In particular, a mouse location_f() could differ between the MOUSE_PRESSED
+  // and MOUSE_RELEASED events. At MOUSE_RELEASED, it will have a targeter()
+  // already cached, while at MOUSE_PRESSED, it will have to calculate it
+  // passing through all  windows, and that could generate rounding error.
+  // std::numeric_limits<float>::epsilon() is not big enough to catch the
+  // rounding error generated by protation.
+  static const float epsilon_scale = 0.0005f;

[Luis Héctor Chávez, 2017/05/17 23:57:11]

nit: constants are usually declared at the very top of the file and with the
convention kMyConstantName (see kLargeCursorScale in L35).

Also, I wonder if it's worth exposing
https://cs.chromium.org/chromium/src/ui/gfx/geometry/quad_f.cc?type=cs&q=eps+...
and addint the epsilon value as template parameter.

[reveman, 2017/05/18 04:21:10]

Yes, kLocatedEventEpsilon above is better. Also, maybe write this as 1.0f /
2000.0f;
Sg, keeping in this file is fine with me as well.

+  return std::abs(event->location_f().x() - location.x()) <= epsilon_scale &&
+         std::abs(event->location_f().y() - location.y()) <= epsilon_scale;
 }
 
 }  // namespace
 
 ////////////////////////////////////////////////////////////////////////////////
 // Pointer, public:
 
 Pointer::Pointer(PointerDelegate* delegate)
     : delegate_(delegate),
       cursor_(ui::CursorType::kNull),
@@ skipping 340 matching lines @@
   if (!root_window)
     return;
 
   aura::client::CursorClient* cursor_client =
       aura::client::GetCursorClient(root_window);
   if (cursor_client)
     cursor_client->SetCursor(cursor_);
 }
 
 }  // namespace exo