Fixes rounding error when calculating MOVE event

components/exo/pointer.cc

Index: components/exo/pointer.cc
diff --git a/components/exo/pointer.cc b/components/exo/pointer.cc
index 073c750192f374b967e3c25061542d8f88136460..3ff0501b82f91d61974d1664aa65354b06b13cf1 100644
--- a/components/exo/pointer.cc
+++ b/components/exo/pointer.cc
@@ -4,6 +4,8 @@
 
 #include "components/exo/pointer.h"
 
+#include <utility>
+
 #include "ash/public/cpp/shell_window_ids.h"
 #include "cc/output/copy_output_request.h"
 #include "cc/output/copy_output_result.h"
@@ -33,6 +35,7 @@ namespace exo {
 namespace {
 
 const float kLargeCursorScale = 2.8f;
+const float kLocatedEventEpsilon = 1.0f / 2000.0f;
 
 // Synthesized events typically lack floating point precision so to avoid
 // generating mouse event jitter we consider the location of these events
@@ -41,7 +44,17 @@ 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, the 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 the hierarchy of windows, and that could
+  // generate rounding error. std::numeric_limits<float>::epsilon() is not big
+  // enough to catch this rounding error.
+  return std::abs(event->location_f().x() - location.x()) <=
+             kLocatedEventEpsilon &&
+         std::abs(event->location_f().y() - location.y()) <=
+             kLocatedEventEpsilon;

[reveman, 2017/05/22 20:12:28]

how about:

gfx::Vector2dF offset = event->location_f() - location;
return offset.LengthSquared() < kLocatedEventEpsilon;

and adjust the Epsilon constant accordingly? I think that might be a bit easier
to read.

 }
 
 }  // namespace