Explicitly suppress scrolling for wheel events that will trigger zooming

content/browser/renderer_host/input/input_router_impl.cc

Index: content/browser/renderer_host/input/input_router_impl.cc
diff --git a/content/browser/renderer_host/input/input_router_impl.cc b/content/browser/renderer_host/input/input_router_impl.cc
index 603adb05821f9e645bbaa97ea9682fc6bf450afc..c19948a55e10d8913e6a5214875a69444d9e52ec 100644
--- a/content/browser/renderer_host/input/input_router_impl.cc
+++ b/content/browser/renderer_host/input/input_router_impl.cc
@@ -136,11 +136,14 @@ void InputRouterImpl::SendWheelEvent(const QueuedWheelEvent& wheel_event) {
     // dropping the old event, as for mouse moves) results in very slow
     // scrolling on the Mac (on which many, very small wheel events are sent).
     // Note that we can't coalesce wheel events for pinches because the GEQ
-    // expects one ACK for each (but it's fine to coalesce non-pinch wheels
-    // into a pinch one).  Note that the GestureEventQueue ensures we only
-    // ever have a single pinch event queued here.
+    // expects one ACK for each.  But such events always have canScroll==false
+    // and hasPreciseDeltas=true, which should never happen for a real wheel
+    // event and so coalescing shouldn't occur.  Note that the

[no sievers, 2015/01/20 20:27:48]

Should there be a DCHECK() for this assumption?

[Rick Byers, 2015/01/21 15:49:13]

Good idea, I think we _should_ be able to say this:

DCHECK((wheel_event.hasPreciseDeltas && !wheel_event.canScroll) ==
wheel_event.synthesized_from_pinch)

Let's try and see if any tests trigger it :-)

[lanwei, 2015/02/04 03:25:58]

Done.

+    // GestureEventQueue ensures we only ever have a single pinch event queued
+    // here.
     if (coalesced_mouse_wheel_events_.empty() ||
-        wheel_event.synthesized_from_pinch ||
+        wheel_event.synthesized_from_pinch !=
+            coalesced_mouse_wheel_events_.back().synthesized_from_pinch ||
         !coalesced_mouse_wheel_events_.back().event.CanCoalesceWith(
             wheel_event.event)) {
       coalesced_mouse_wheel_events_.push_back(wheel_event);
@@ -455,7 +458,7 @@ void InputRouterImpl::SendSyntheticWheelEventForPinch(
   wheelEvent.wheelTicksX = 0;
   wheelEvent.wheelTicksY =
       pinch_event.event.data.pinchUpdate.scale > 1 ? 1 : -1;
-
+  wheelEvent.canScroll = false;
   SendWheelEvent(QueuedWheelEvent(
       MouseWheelEventWithLatencyInfo(wheelEvent, pinch_event.latency), true));
 }