Do not scale ScrollEvent by device scale factor

We used to scale scroll events in event_transformation_handler
because we want to mimic the behaviour of the CrOS X patch which
scales pointer motions w.r.t. the screen resolution. See the
patch here https://chromium-review.googlesource.com/#/c/45522/.
The idea is actually wrong because Aura Views use DIPs instead of
pixels as what X server does. The scaling would make scroll events
to have 2x higher speed on Pixel as compared to normal resolution
devices. This CL fixes the problem so that scroll events will
have identical sensitivity on all devices regardless of the screen
resolution.

Note that the sensitivity would still be slightly different due to
the sllightly different DIP resolution on each device.

Contributed by sheckylin@chromium.org

BUG=288211
TEST=Tested on Pixel and non-Pixel devices. Make sure:
1. Scrolling on webpages with touchpads goes through roughly the same
amount of page if the scrolling distance and speed is the same.
2. 2f back/forward gesture should require roughly equal distance
of scrolling on the touchpad at the same speed to trigger.
3. 3f tab-switching should go through roughly the same ratio of tab
counts for the equal distance of scrolling on the touchpad at the same
speed.

Review URL: https://chromiumcodereview.appspot.com/23452037

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224341 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 5 insertions, 8 deletions

diff --git a/ash/display/event_transformation_handler.cc b/ash/display/event_transformation_handler.cc
index 5f9e450..aaac56c 100644
--- a/ash/display/event_transformation_handler.cc
+++ b/ash/display/event_transformation_handler.cc
@@ -40,20 +40,17 @@ void EventTransformationHandler::OnScrollEvent(ui::ScrollEvent* event) {
   if (transformation_mode_ == TRANSFORM_NONE)
     return;
 
-  // Get the device scale factor and stack it on the final scale factor.
+  // It is unnecessary to scale the event for the device scale factor since
+  // the event locations etc. are already in DIP.
   gfx::Point point_in_screen(event->location());
   aura::Window* target = static_cast<aura::Window*>(event->target());
-  const float scale_at_target = ui::GetDeviceScaleFactor(target->layer());
-  float scale = scale_at_target;
-
-  // Apply some additional scaling if the display is non-integrated.
   wm::ConvertPointToScreen(target, &point_in_screen);
   const gfx::Display& display =
       Shell::GetScreen()->GetDisplayNearestPoint(point_in_screen);
-  if (!display.IsInternal())
-    scale *= kBoostForNonIntegrated;
 
-  event->Scale(scale);
+  // Apply some additional scaling if the display is non-integrated.
+  if (!display.IsInternal())
+    event->Scale(kBoostForNonIntegrated);
 }
 
 #if defined(OS_CHROMEOS)