exo: Replace fling cancel with generic zero distance scroll

components/exo/pointer.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/exo/pointer.h"
 
 #include "ash/public/cpp/shell_window_ids.h"
 #include "cc/output/copy_output_request.h"
 #include "cc/output/copy_output_result.h"
 #include "components/exo/pointer_delegate.h"
@@ skipping 189 matching lines @@
       break;
     }
     case ui::ET_MOUSEWHEEL: {
       delegate_->OnPointerScroll(
           event->time_stamp(),
           static_cast<ui::MouseWheelEvent*>(event)->offset(), true);
       delegate_->OnPointerFrame();
       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:
     case ui::ET_MOUSE_DRAGGED:
     case ui::ET_MOUSE_ENTERED:
     case ui::ET_MOUSE_EXITED:
     case ui::ET_MOUSE_CAPTURE_CHANGED:
       break;
     default:
       NOTREACHED();
       break;
   }
 
+  last_event_type_ = event->type();
   UpdateCursorScale();
 }
 
 void Pointer::OnScrollEvent(ui::ScrollEvent* event) {
   OnMouseEvent(event);
 }
 
 ////////////////////////////////////////////////////////////////////////////////
 // WMHelper::CursorObserver overrides:
 
@@ skipping 142 matching lines @@
   if (!root_window)
     return;
 
   aura::client::CursorClient* cursor_client =
       aura::client::GetCursorClient(root_window);
   if (cursor_client)
     cursor_client->SetCursor(cursor_);
 }
 
 }  // namespace exo