Scaling scroll/fling events in multi-monitor setup

The CL adds an event filter that transforms input event properties
in the extended desktop environment. It currently scales scroll/fling
events' offset values based on the following logic:

1. Linear scaling w.r.t. the device scale factor, which is 2x for
   highDPI displays.
2. 1.2x scaling for external displays, since they are usually larger
   than internal ones.

Both scalings can stack on each other, i.e. you can get as high as
a 2.4x boost if you have a high DPI external display.

Contributed by sheckylin@chromium.org

BUG=chromium:166392
TEST=Tested on link w/o external display.

Change-Id: Ie29e03fcf9f6e5ae1dd6231990103237c3129fd6


Review URL: https://chromiumcodereview.appspot.com/11743013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@176038 0039d316-1c4b-4281-b951-d872f2087c98

2 files changed, 10 insertions, 0 deletions

diff --git a/ui/base/events/event.cc b/ui/base/events/event.cc
index e999f33..69794cf 100644
--- a/ui/base/events/event.cc
+++ b/ui/base/events/event.cc
@@ -639,6 +639,11 @@ ScrollEvent::ScrollEvent(EventType type,
   CHECK(IsScrollEvent());
 }
 
+void ScrollEvent::Scale(const float factor) {
+  x_offset_ *= factor;
+  y_offset_ *= factor;
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // GestureEvent
 
diff --git a/ui/base/events/event.h b/ui/base/events/event.h
index a5d2887..574cb7f 100644
--- a/ui/base/events/event.h
+++ b/ui/base/events/event.h
@@ -631,6 +631,11 @@ class UI_EXPORT ScrollEvent : public MouseEvent {
               float x_offset,
               float y_offset);
 
+  // Scale the scroll event's offset value.
+  // This is useful in the multi-monitor setup where it needs to be scaled
+  // to provide a consistent user experience.
+  void Scale(const float factor);
+
   float x_offset() const { return x_offset_; }
   float y_offset() const { return y_offset_; }
   int finger_count() const { return finger_count_; }