Unify Android Webview and Chrome's fling

content/renderer/input/input_handler_proxy.cc

Index: content/renderer/input/input_handler_proxy.cc
diff --git a/content/renderer/input/input_handler_proxy.cc b/content/renderer/input/input_handler_proxy.cc
index f714ef1f3d956678b8ba0b3cd744f85715a142a7..535288761060daa8635b41c1826df4b5492e47e5 100644
--- a/content/renderer/input/input_handler_proxy.cc
+++ b/content/renderer/input/input_handler_proxy.cc
@@ -416,7 +416,9 @@ InputHandlerProxy::EventDisposition InputHandlerProxy::HandleMouseWheel(
 
 InputHandlerProxy::EventDisposition InputHandlerProxy::HandleGestureScrollBegin(
     const WebGestureEvent& gesture_event) {
-  DCHECK(!gesture_scroll_on_impl_thread_);
+  if (gesture_scroll_on_impl_thread_)

[jdduke (slow), 2015/05/04 22:02:10]

I'm surprised this was necessary, was there a particular gesture sequence where
we didn't receive a GestureFlingCancel first?

[hush (inactive), 2015/05/05 21:15:32]

This is not necessary in chrome code I think.
In Android WebView, you could fling the webview with the finger immediately
after calling the Java API "WebView#flingScroll()", before the synthesized fling
finishes. I hit this DCHECK in that case.

When 2 real flings go back to back, CancelCurrentFlingWithoutNotifyingClient
will always be called before the 2nd fling. But when there is a real fling
immediately following a synthesized fling,
CancelCurrentFlingWithoutNotifyingClient is sometimes not called before the real
fling. And that is the problem. I got around it by calling "CancelCurrentFling"
in this case.
The better solution would be to figure out why
CancelCurrentFlingWithoutNotifyingClient is not called in that case. I don't
know it yet.

+    CancelCurrentFling();
+
 #ifndef NDEBUG
   DCHECK(!expect_scroll_update_end_);
   expect_scroll_update_end_ = true;
@@ -528,7 +530,7 @@ InputHandlerProxy::EventDisposition InputHandlerProxy::HandleGestureFlingStart(
           WebPoint(gesture_event.globalX, gesture_event.globalY);
       fling_parameters_.modifiers = gesture_event.modifiers;
       fling_parameters_.sourceDevice = gesture_event.sourceDevice;
-      input_handler_->SetNeedsAnimate();
+      input_handler_->SetNeedsAnimateInput();
       return DID_HANDLE;
     }
     case cc::InputHandler::SCROLL_UNKNOWN:
@@ -750,7 +752,7 @@ void InputHandlerProxy::Animate(base::TimeTicks time) {
         monotonic_time_sec >= fling_parameters_.startTime +
                                   kMaxSecondsFromFlingTimestampToFirstAnimate) {
       fling_parameters_.startTime = monotonic_time_sec;
-      input_handler_->SetNeedsAnimate();
+      input_handler_->SetNeedsAnimateInput();
       return;
     }
   }
@@ -763,7 +765,7 @@ void InputHandlerProxy::Animate(base::TimeTicks time) {
     fling_is_active = false;
 
   if (fling_is_active) {
-    input_handler_->SetNeedsAnimate();
+    input_handler_->SetNeedsAnimateInput();
   } else {
     TRACE_EVENT_INSTANT0("input",
                          "InputHandlerProxy::animate::flingOver",