Reset scroll status when a fling is ignored

In WebView, flings that target the root scrolling layer are ignored by
the compositor. In this case, we failed to clean up the scroll state in
InputHandlerProxy, leading it to believe that a scroll was still active
after the fling had been ignored. Handle this case, resetting the scroll
activity bit when a fling is ignored.

BUG=469369

Review URL: https://codereview.chromium.org/1028533004

Cr-Commit-Position: refs/heads/master@{#321932}

content/renderer/input/input_handler_proxy.cc

@@ -537,6 +537,7 @@ InputHandlerProxy::EventDisposition InputHandlerProxy::HandleGestureFlingStart(
                            "InputHandlerProxy::HandleGestureFling::"
                            "scroll_on_main_thread",
                            TRACE_EVENT_SCOPE_THREAD);
+      gesture_scroll_on_impl_thread_ = false;
       fling_may_be_active_on_main_thread_ = true;
       return DID_NOT_HANDLE;
     }
@@ -545,6 +546,7 @@ InputHandlerProxy::EventDisposition InputHandlerProxy::HandleGestureFlingStart(
           "input",
           "InputHandlerProxy::HandleGestureFling::ignored",
           TRACE_EVENT_SCOPE_THREAD);
+      gesture_scroll_on_impl_thread_ = false;
       if (gesture_event.sourceDevice == blink::WebGestureDeviceTouchpad) {
         // We still pass the curve to the main thread if there's nothing
         // scrollable, in case something

content/renderer/input/input_handler_proxy_unittest.cc

@@ -961,6 +961,8 @@ TEST_F(InputHandlerProxyTest, GestureFlingIgnoredTouchscreen) {
   expected_disposition_ = InputHandlerProxy::DROP_EVENT;
   VERIFY_AND_RESET_MOCKS();
 
+  // Flings ignored by the InputHandler should be dropped, signalling the end
+  // of the touch scroll sequence.
   EXPECT_CALL(mock_input_handler_, FlingScrollBegin())
       .WillOnce(testing::Return(cc::InputHandler::SCROLL_IGNORED));
 
@@ -970,9 +972,12 @@ TEST_F(InputHandlerProxyTest, GestureFlingIgnoredTouchscreen) {
 
   VERIFY_AND_RESET_MOCKS();
 
-  // Even if we didn't start a fling ourselves, we still need to send the cancel
-  // event to the widget.
-  gesture_.type = WebInputEvent::GestureFlingCancel;
+  // Subsequent scrolls should behave normally, even without an intervening
+  // GestureFlingCancel, as the original GestureFlingStart was dropped.
+  expected_disposition_ = InputHandlerProxy::DID_HANDLE;
+  EXPECT_CALL(mock_input_handler_, ScrollBegin(testing::_, testing::_))
+      .WillOnce(testing::Return(cc::InputHandler::SCROLL_STARTED));
+  gesture_.type = WebInputEvent::GestureScrollBegin;
   gesture_.sourceDevice = blink::WebGestureDeviceTouchscreen;
   EXPECT_EQ(expected_disposition_, input_handler_->HandleInputEvent(gesture_));
 }