overscroll: Do not overscroll if the beginning of a scroll was consumed.

content/browser/renderer_host/overscroll_controller.cc

Index: content/browser/renderer_host/overscroll_controller.cc
diff --git a/content/browser/renderer_host/overscroll_controller.cc b/content/browser/renderer_host/overscroll_controller.cc
index 14a82e6fe9460b916bcf12559ba1caef9755cabd..b6cafe1adfcdd3a6704529340c8144ba929c2a3c 100644
--- a/content/browser/renderer_host/overscroll_controller.cc
+++ b/content/browser/renderer_host/overscroll_controller.cc
@@ -16,6 +16,7 @@ OverscrollController::OverscrollController(
     RenderWidgetHostImpl* render_widget_host)
     : render_widget_host_(render_widget_host),
       overscroll_mode_(OVERSCROLL_NONE),
+      scroll_state_(STATE_UNKNOWN),
       overscroll_delta_x_(0.f),
       overscroll_delta_y_(0.f),
       delegate_(NULL) {
@@ -27,6 +28,25 @@ OverscrollController::~OverscrollController() {
 bool OverscrollController::WillDispatchEvent(
     const WebKit::WebInputEvent& event,
     const cc::LatencyInfo& latency_info) {
+  if (scroll_state_ != STATE_UNKNOWN) {
+    if (event.type == WebKit::WebInputEvent::GestureScrollEnd) {
+      scroll_state_ = STATE_UNKNOWN;
+    } else if (event.type == WebKit::WebInputEvent::GestureFlingStart) {
+      // Start of a fling indicates the end of a scroll, both from touchpad and
+      // touchscreen. So it is not necessary to check |sourceDevice| of the
+      // event.
+      scroll_state_ = STATE_UNKNOWN;
+    } else if (event.type == WebKit::WebInputEvent::MouseWheel) {
+      const WebKit::WebMouseWheelEvent& wheel =
+          static_cast<const WebKit::WebMouseWheelEvent&>(event);
+      if (wheel.hasPreciseScrollingDeltas &&
+          (wheel.phase == WebKit::WebMouseWheelEvent::PhaseEnded ||
+           wheel.phase == WebKit::WebMouseWheelEvent::PhaseCancelled)) {
+        scroll_state_ = STATE_UNKNOWN;
+      }
+    }
+  }
+
   if (DispatchEventCompletesAction(event)) {
     CompleteAction();
 
@@ -79,14 +99,24 @@ bool OverscrollController::WillDispatchEvent(
 
 void OverscrollController::ReceivedEventACK(const WebKit::WebInputEvent& event,
                                             bool processed) {
-  if (processed)
+  if (processed) {
+    // If a scroll event is consumed by the page, i.e. some content on the page
+    // has been scrolled, then there is not going to be an overscroll gesture,
+    // until the current scroll ends, and a new scroll gesture starts.
+    if (scroll_state_ == STATE_UNKNOWN &&
+        (event.type == WebKit::WebInputEvent::MouseWheel ||
+         event.type == WebKit::WebInputEvent::GestureScrollUpdate)) {
+      scroll_state_ = STATE_CONTENT_SCROLLING;
+    }
     return;
+  }
   ProcessEventForOverscroll(event);
 }
 
 void OverscrollController::Reset() {
   overscroll_mode_ = OVERSCROLL_NONE;
   overscroll_delta_x_ = overscroll_delta_y_ = 0.f;
+  scroll_state_ = STATE_UNKNOWN;
 }
 
 bool OverscrollController::DispatchEventCompletesAction (
@@ -177,10 +207,11 @@ void OverscrollController::ProcessEventForOverscroll(
     case WebKit::WebInputEvent::MouseWheel: {
       const WebKit::WebMouseWheelEvent& wheel =
           static_cast<const WebKit::WebMouseWheelEvent&>(event);
-      if (wheel.hasPreciseScrollingDeltas) {
-        ProcessOverscroll(wheel.deltaX * wheel.accelerationRatioX,
-                          wheel.deltaY * wheel.accelerationRatioY);
-      }
+      if (!wheel.hasPreciseScrollingDeltas)
+        return;
+
+      ProcessOverscroll(wheel.deltaX * wheel.accelerationRatioX,
+                        wheel.deltaY * wheel.accelerationRatioY);
       break;
     }
     case WebKit::WebInputEvent::GestureScrollUpdate: {
@@ -223,6 +254,8 @@ void OverscrollController::ProcessEventForOverscroll(
 }
 
 void OverscrollController::ProcessOverscroll(float delta_x, float delta_y) {
+  if (scroll_state_ == STATE_CONTENT_SCROLLING)
+    return;
   overscroll_delta_x_ += delta_x;
   overscroll_delta_y_ += delta_y;
 
@@ -292,6 +325,8 @@ void OverscrollController::SetOverscrollMode(OverscrollMode mode) {
   overscroll_mode_ = mode;
   if (overscroll_mode_ == OVERSCROLL_NONE)
     overscroll_delta_x_ = overscroll_delta_y_ = 0.f;
+  else
+    scroll_state_ = STATE_OVERSCROLLING;
   if (delegate_)
     delegate_->OnOverscrollModeChange(old_mode, overscroll_mode_);
 }