[Android] Always precede Tap gesture events with TapDown

content/browser/android/content_view_core_impl.cc

Index: content/browser/android/content_view_core_impl.cc
diff --git a/content/browser/android/content_view_core_impl.cc b/content/browser/android/content_view_core_impl.cc
index b2c2f7a48d3e0e0017b025b7cd27d20ff7018ad9..4697f6b00858eda59fe55c580a22713cff2d7c28 100644
--- a/content/browser/android/content_view_core_impl.cc
+++ b/content/browser/android/content_view_core_impl.cc
@@ -1028,11 +1028,17 @@ void ContentViewCoreImpl::FlingCancel(JNIEnv* env, jobject obj, jlong time_ms) {
 
 void ContentViewCoreImpl::SingleTap(JNIEnv* env, jobject obj, jlong time_ms,
                                     jfloat x, jfloat y) {
-  WebGestureEvent event = MakeGestureEvent(
+  // Tap gestures should always be preceded by a TapDown, ensuring consistency
+  // with the touch-based gesture detection pipeline.
+  WebGestureEvent tap_down_event = MakeGestureEvent(
+      WebInputEvent::GestureTapDown, time_ms, x, y);

[Rick Byers, 2014/11/17 22:55:24]

What about GestureShowPress?  Shouldn't Tap always be preceded by that too?

[jdduke (slow), 2014/11/17 23:45:21]

Hmm, you tell me :) I believe we insert ShowPress before Tap in the
TouchDispositionGestureFilter, but I don't think we validate the ordering
anywhere, and I'm not sure if we should (we can if it's worth it). I guess I
don't understand the side effects of TapDown+ShowPress well enough to know which
is critical. Inserting ShowPress before Tap has always felt weird =/ (i.e., why
can't Blink do it implicitly?).

[Zeeshan Qureshi, 2014/11/17 23:49:06]

I believe Tap is only fired when the finger lifts? We could hypothetically use
TapDown to generate ShowPress though. I'm not sure what the browser-side
contract is for the two.

[jdduke (slow), 2014/11/17 23:52:20]

Well, I think we still want the delay before activating ShowPress (to
accommodate scrolling). For gesture detection, we'll either get a showpress or a
scroll, never both, and if the press event doesn't fire before the tap event, we
insert the press event.

+  tap_down_event.data.tap.tapCount = 1;
+  SendGestureEvent(tap_down_event);
+
+  WebGestureEvent tap_event = MakeGestureEvent(
       WebInputEvent::GestureTap, time_ms, x, y);
-  event.data.tap.tapCount = 1;
-
-  SendGestureEvent(event);
+  tap_event.data.tap.tapCount = 1;
+  SendGestureEvent(tap_event);
 }
 
 void ContentViewCoreImpl::DoubleTap(JNIEnv* env, jobject obj, jlong time_ms,