Consuming a touch move prevents only the next scroll update.

content/browser/renderer_host/input/touch_disposition_gesture_filter.h

Index: content/browser/renderer_host/input/touch_disposition_gesture_filter.h
diff --git a/content/browser/renderer_host/input/touch_disposition_gesture_filter.h b/content/browser/renderer_host/input/touch_disposition_gesture_filter.h
index f2acc3b47c1b1ab6001cd16fa4f1315264baa3a3..7680aed196e34e2154ed45cbafe7c3d5e030416d 100644
--- a/content/browser/renderer_host/input/touch_disposition_gesture_filter.h
+++ b/content/browser/renderer_host/input/touch_disposition_gesture_filter.h
@@ -5,14 +5,26 @@
 #ifndef CONTENT_BROWSER_RENDERER_HOST_INPUT_TOUCH_DISPOSITION_GESTURE_FILTER_H_
 #define CONTENT_BROWSER_RENDERER_HOST_INPUT_TOUCH_DISPOSITION_GESTURE_FILTER_H_
 
-#include <deque>
+#include <map>

[Rick Byers, 2014/02/13 21:59:25]

doesn't look like you're actually using map here...

[tdresser, 2014/02/14 13:52:42]

Done.

 #include <queue>
+#include <set>
 
 #include "content/browser/renderer_host/input/gesture_event_packet.h"
 #include "content/common/content_export.h"
 #include "content/port/common/input_event_ack_state.h"
 #include "third_party/WebKit/public/web/WebInputEvent.h"
 
+namespace {
+
+enum RequiredTouches {
+  RT_NONE    = 0,
+  RT_START   = 1 << 0,
+  RT_MOVE    = 1 << 1,

[Rick Byers, 2014/02/13 21:59:25]

As discussed now in person and in the doc, I think we can eliminate the MOVE
state (treat it just like CURRENT).

[tdresser, 2014/02/14 13:52:42]

Done.

+  RT_CURRENT = 1 << 2,
+};
+
+}  // namespace
+
 namespace content {
 
 // Interface with which the |TouchDispositionGestureFilter| forwards gestures
@@ -56,30 +68,42 @@ class CONTENT_EXPORT TouchDispositionGestureFilter {
   // generated by timeouts from a statinoary finger.
   class GestureSequence {
    public:
+    struct GestureHandlingState {
+      GestureHandlingState();
+      bool seen_ack;

[Rick Byers, 2014/02/13 21:59:25]

Since the precise definition of these bits are critical to the behavior, I think
it's worth adding comments for each that precisely define their semantics.
start_consumed/move_consumed are pretty obvious, but the exact semantics of
seen_ack and no_consumer aren't obvious to me before looking at the code.

[tdresser, 2014/02/14 13:52:42]

Done.

+      bool start_consumed;
+      bool move_consumed;
+      bool no_consumer;
+    };
+
     GestureSequence();
     ~GestureSequence();
 
     void Push(const GestureEventPacket& packet);
     void Pop();
     const GestureEventPacket& Front() const;
-    void UpdateState(InputEventAckState ack_state);
-    bool IsGesturePrevented() const;
+    void UpdateState(GestureEventPacket::GestureSource gesture_source,
+                     InputEventAckState ack_state);
     bool IsEmpty() const;
+    const GestureHandlingState& state() const { return state_; };
 
    private:
     std::queue<GestureEventPacket> packets_;
-    enum GestureHandlingState {
-      PENDING,                 // The sequence has yet to receive an ack.
-      ALLOWED_UNTIL_PREVENTED, // Gestures in the sequence are allowed until
-                               // a source touch is preventDefault'ed.
-      ALWAYS_ALLOWED,          // All remaining sequence gestures are forwarded.
-      ALWAYS_PREVENTED         // All remaining sequence gestures are dropped.
-    };
     GestureHandlingState state_;
   };
+  bool IsGesturePrevented(blink::WebInputEvent::Type type,
+                          InputEventAckState current,
+                          const GestureSequence::GestureHandlingState& state)
+      const;
+
   void UpdateAndDispatchPackets(GestureSequence* sequence,
                                 InputEventAckState ack_result);
-  void SendPacket(const GestureEventPacket& packet);
+
+  void FilterAndSendPacket(
+      const GestureEventPacket& packet,
+      const GestureSequence::GestureHandlingState& sequence_state,
+      InputEventAckState ack_state);
+
   void SendGesture(const blink::WebGestureEvent& gesture);
   void CancelTapIfNecessary();
   void CancelFlingIfNecessary();
@@ -89,6 +113,9 @@ class CONTENT_EXPORT TouchDispositionGestureFilter {
   TouchDispositionGestureFilterClient* client_;
   std::queue<GestureSequence> sequences_;
 
+  std::set<blink::WebInputEvent::Type>
+      last_event_of_type_dropped_;

[Rick Byers, 2014/02/13 21:59:25]

please add a comment describing the precise semantics.  In particular, should we
expect this set to be cleared at the start of every touch sequence?

[tdresser, 2014/02/14 13:52:42]

Done. I previously wasn't clearing this at the start of a new touch sequence,
and I don't think it's strictly necessary, but it does make things easier to
reason about, and there's definitely room for potential bugs if it isn't
cleared.

+
   // Bookkeeping for inserting synthetic Gesture{Tap,Fling}Cancel events
   // when necessary, e.g., GestureTapCancel when scrolling begins, or
   // GestureFlingCancel when a user taps following a GestureFlingStart.