Slop region check for multi-finger scroll

ui/events/gesture_detection/gesture_provider.cc

Index: ui/events/gesture_detection/gesture_provider.cc
diff --git a/ui/events/gesture_detection/gesture_provider.cc b/ui/events/gesture_detection/gesture_provider.cc
index c1d3163cfd6f6d14728b5289daa3d805e2e9b2c4..3a2e1add71f91b7bcc0efd32df122c0abe765bd4 100644
--- a/ui/events/gesture_detection/gesture_provider.cc
+++ b/ui/events/gesture_detection/gesture_provider.cc
@@ -118,7 +118,6 @@ class GestureProvider::GestureListenerImpl : public ScaleGestureListener,
       tap_down_point_ = gfx::PointF(event.GetX(), event.GetY());
       max_diameter_before_show_press_ = event.GetTouchMajor();
     }
-
     gesture_detector_.OnTouchEvent(event);
     scale_gesture_detector_.OnTouchEvent(event);
 
@@ -288,18 +287,17 @@ class GestureProvider::GestureListenerImpl : public ScaleGestureListener,
                 float raw_distance_y) override {
     float distance_x = raw_distance_x;
     float distance_y = raw_distance_y;
-    if (!scroll_event_sent_ && e2.GetPointerCount() == 1) {
-      // Remove the touch slop region from the first scroll event to
-      // avoid a jump. Touch slop isn't used for multi-finger
-      // gestures, so in those cases we don't subtract the slop.
+    if (!scroll_event_sent_ && e2.GetPointerCount() < 3) {
+      // Remove the touch slop region from the first scroll event to avoid a
+      // jump. Touch slop isn't used for scroll gestures with greater than 2
+      // pointers down, in those cases we don't subtract the slop.
       float distance =
           std::sqrt(distance_x * distance_x + distance_y * distance_y);
       float epsilon = 1e-3f;
       if (distance > epsilon) {
-        float ratio =
-            std::max(0.f,
-                     distance - config_.gesture_detector_config.touch_slop) /
-            distance;
+        float subtraction_value = config_.gesture_detector_config.touch_slop /
+                                  e2.GetPointerCount();
+        float ratio = std::max(0.f, distance - subtraction_value ) / distance;

[tdresser, 2016/06/10 19:34:57]

Is this logic correct? If the two pointers move in opposite directions and
exceed their slop regions, and then both scroll 1px down, what will happen?

[sahel, 2016/06/23 22:34:44]

Done.

         distance_x *= ratio;
         distance_y *= ratio;
       }