exo: Replace fling cancel with generic zero distance scroll

components/exo/pointer.cc

Index: components/exo/pointer.cc
diff --git a/components/exo/pointer.cc b/components/exo/pointer.cc
index e650b4f4ffe35b7732e406cdf59a315515d966a9..3473ae8a7e510d8b8051e81935b3d46d4907bf33 100644
--- a/components/exo/pointer.cc
+++ b/components/exo/pointer.cc
@@ -207,13 +207,28 @@ void Pointer::OnMouseEvent(ui::MouseEvent* event) {
       break;
     }
     case ui::ET_SCROLL_FLING_START: {
+      // Fling start in chrome signals the lifting of fingers after scrolling.
+      // In wayland terms this signals the end of a scroll sequence.
       delegate_->OnPointerScrollStop(event->time_stamp());
       delegate_->OnPointerFrame();
       break;
     }
     case ui::ET_SCROLL_FLING_CANCEL: {
-      delegate_->OnPointerScrollCancel(event->time_stamp());
-      delegate_->OnPointerFrame();
+      // Fling cancel is generated very generously at every touch of the
+      // touchpad. Since it's not directly supported by wayland, we do not

[reveman, 2017/05/03 21:02:49]

nit: we shouldn't be referring to wayland in this code. s/wayland/delegate/ is
fine.

+      // want limit this event to only right after a fling start has been
+      // generated to prevent erronous behavior.
+      if (last_event_type_ == ui::ET_SCROLL_FLING_START) {
+        // Since wayland does not have support for fling cancel events

[reveman, 2017/05/03 21:02:48]

nit: s/wayland/delegate/

+        // (which are supposed to stop any kind of fling/kinetic scrolling
+        // motion), we emulate this by starting a new scroll sequence that
+        // scrolls by 0 pixels, effectively stopping any kinetic scroll motion.
+        delegate_->OnPointerScroll(event->time_stamp(), gfx::Vector2dF(),
+                                   false);
+        delegate_->OnPointerFrame();
+        delegate_->OnPointerScrollStop(event->time_stamp());
+        delegate_->OnPointerFrame();
+      }
       break;
     }
     case ui::ET_MOUSE_MOVED:
@@ -227,6 +242,7 @@ void Pointer::OnMouseEvent(ui::MouseEvent* event) {
       break;
   }
 
+  last_event_type_ = event->type();
   UpdateCursorScale();
 }