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 ef06cb94e5b726a900703cbad0f43b1ba08c5462..1cc32a426376b4baeb219c431323cf15b476a977 100644
--- a/components/exo/pointer.cc
+++ b/components/exo/pointer.cc
@@ -206,13 +206,26 @@ 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 the delegate, we do not
+      // 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) {
+        // We emulate fling cancel 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:
@@ -226,6 +239,7 @@ void Pointer::OnMouseEvent(ui::MouseEvent* event) {
       break;
   }
 
+  last_event_type_ = event->type();
   UpdateCursorScale();
 }