Ink drops with touch input are redundant on Windows since the OS does something similar

ui/views/animation/ink_drop_host_view.cc

 // Copyright 2016 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 "ui/views/animation/ink_drop_host_view.h"
 
 #include "ui/events/event.h"
 #include "ui/events/scoped_target_handler.h"
 #include "ui/gfx/color_palette.h"
 #include "ui/gfx/geometry/size_conversions.h"
@@ skipping 154 matching lines @@
 }
 
 gfx::Point InkDropHostView::GetInkDropCenterBasedOnLastEvent() const {
   return last_ripple_triggering_event_
              ? last_ripple_triggering_event_->location()
              : GetLocalBounds().CenterPoint();
 }
 
 void InkDropHostView::AnimateInkDrop(InkDropState state,
                                      const ui::LocatedEvent* event) {
+#if defined(OS_WIN)
+  // The reason for this code is to ensure the ink-drop never gets "stuck"

[Evan Stade, 2016/06/27 15:19:46]

I think you're skipping ahead a bit here, imo the right lead would be "Don't
initiate ink drops for touch events on Windows." Also I'd much prefer if you
didn't copy the code into the comments but integrated the code and comments
instead. How about this:

// Don't create new ink drops for touch events on Windows. However, touch events
must dismiss existing ink drop animations.
if (event && event->IsGestureEvent()) {
  // The ink-drop should never transition to a "pending" state from a touch
  // event.
  if (state == InkDropState::ACTION_PENDING || state ==
InkDropState::ALTERNATE_ACTION_PENDING)
    return;

  // If the state is already "pending", presumably from a non-touch event such
  // as mouse or keyboard, then a triggered action state should be allowed.
Conversely if the state is already targeted to hidden then the touch should be
ignored.
  if ((state == InkDropState::ACTION_TRIGGERED ||
      state == InkDropState::ALTERNATE_ACTION_TRIGGERED) &&
      ink_drop_->GetTargetInkDropState() == InkDropState::HIDDEN)
    return;
}

+  // because of mismatched events or states.
+  //
+  // The ink-drop should never transition to a "pending" state from a touch
+  // event. This is the first part of the conditional:
+  //
+  // state == InkDropState::ACTION_PENDING ||
+  // state == InkDropState::ALTERNATE_ACTION_PENDING
+  //
+  // If the state is already "pending", presumably, from a non-touch event such
+  // as mouse or keyboard, then a triggered action state should be allowed. The
+  // second part of the conditional:
+  //
+  // (state == InkDropState::ACTION_TRIGGERED ||
+  //  state == InkDropState::ALTERNATE_ACTION_TRIGGERED) &&
+  // ink_drop_->GetTargetInkDropState() == InkDropState::HIDDEN
+  //
+  // Finally, all of the above is predicated on the event itself being a
+  // gesture/touch event:
+  //
+  // event && event->IsGestureEvent()
+  //
+  if ((state == InkDropState::ACTION_PENDING ||
+       state == InkDropState::ALTERNATE_ACTION_PENDING ||
+       ((state == InkDropState::ACTION_TRIGGERED ||
+         state == InkDropState::ALTERNATE_ACTION_TRIGGERED) &&
+        ink_drop_->GetTargetInkDropState() == InkDropState::HIDDEN)) &&
+      event && event->IsGestureEvent())
+    return;
+#endif
   last_ripple_triggering_event_.reset(
       event ? ui::Event::Clone(*event).release()->AsLocatedEvent() : nullptr);
   ink_drop_->AnimateToState(state);
 }
 
 void InkDropHostView::VisibilityChanged(View* starting_from, bool is_visible) {
   View::VisibilityChanged(starting_from, is_visible);
   if (GetWidget() && !is_visible) {
     ink_drop()->AnimateToState(InkDropState::HIDDEN);
     ink_drop()->SetHovered(false);
@@ skipping 39 matching lines @@
   if (has_an_ink_drop) {
     ink_drop_.reset(new InkDropImpl(this));
     gesture_handler_.reset(new InkDropGestureHandler(this, ink_drop_.get()));
   } else {
     gesture_handler_.reset();
     ink_drop_.reset(new InkDropStub());
   }
 }
 
 }  // namespace views