Clarify devtools console log when a passive event listener is preventDefaulted.

third_party/WebKit/Source/core/events/Event.cpp

 /*
  * Copyright (C) 2001 Peter Kelly (pmk@post.com)
  * Copyright (C) 2001 Tobias Anton (anton@stud.fbi.fh-darmstadt.de)
  * Copyright (C) 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2003, 2005, 2006, 2008 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 62 matching lines @@
     , m_cancelable(cancelableArg)
     , m_composed(composedMode == ComposedMode::Composed)
     , m_isEventTypeScopedInV0(isEventTypeScopedInV0(eventType))
     , m_propagationStopped(false)
     , m_immediatePropagationStopped(false)
     , m_defaultPrevented(false)
     , m_defaultHandled(false)
     , m_cancelBubble(false)
     , m_wasInitialized(true)
     , m_isTrusted(false)
-    , m_handlingPassive(false)
     , m_preventDefaultCalledOnUncancelableEvent(false)
+    , m_handlingPassive(PassiveMode::NotPassive)
     , m_eventPhase(0)
     , m_currentTarget(nullptr)
     , m_platformTimeStamp(platformTimeStamp)
 {
 }
 
 Event::Event(const AtomicString& eventType, const EventInit& initializer)
     : Event(eventType, initializer.bubbles(), initializer.cancelable(), initializer.composed() ? ComposedMode::Composed : ComposedMode::Scoped, monotonicallyIncreasingTime())
 {
 }
@@ skipping 123 matching lines @@
     return false;
 }
 
 bool Event::isBeforeUnloadEvent() const
 {
     return false;
 }
 
 void Event::preventDefault()
 {
-    if (m_handlingPassive) {
+    if (m_handlingPassive != PassiveMode::NotPassive) {
         m_preventDefaultCalledDuringPassive = true;
+
         const LocalDOMWindow* window = m_eventPath ? m_eventPath->windowEventContext().window() : 0;
-        if (window)
-            window->printErrorMessage("Unable to preventDefault inside passive event listener invocation.");
+        if (window) {
+            const char* devToolsMsg = nullptr;
+            switch (m_handlingPassive) {
+            case PassiveMode::NotPassive:
+                break;

[bokan, 2016/08/25 13:19:44]

perhaps this should be an ASSERT_NOT_REACHED?

[dtapuska, 2016/08/25 14:02:57]

done; used NOTREACHED as ASSERT_NOT_REACHED is deprecated.

+            case PassiveMode::Passive:
+                devToolsMsg = "Unable to preventDefault inside passive event listener invocation.";
+                break;
+            case PassiveMode::PassiveForcedDocumentLevel:
+                devToolsMsg = "Unable to preventDefault inside passive event listener due to target being treated as passive. See https://www.chromestatus.com/features/5093566007214080";
+                break;
+            }
+            if (devToolsMsg)
+                window->printErrorMessage(devToolsMsg);
+        }
         return;
     }
 
     if (m_cancelable)
         m_defaultPrevented = true;
     else
         m_preventDefaultCalledOnUncancelableEvent = true;
 }
 
 void Event::setTarget(EventTarget* target)
@@ skipping 31 matching lines @@
 HeapVector<Member<EventTarget>> Event::path(ScriptState* scriptState) const
 {
     return pathInternal(scriptState, NonEmptyAfterDispatch);
 }
 
 HeapVector<Member<EventTarget>> Event::composedPath(ScriptState* scriptState) const
 {
     return pathInternal(scriptState, EmptyAfterDispatch);
 }
 
-void Event::setHandlingPassive(bool value)
+void Event::setHandlingPassive(PassiveMode mode)
 {
-    m_handlingPassive = value;
+    m_handlingPassive = mode;
     m_preventDefaultCalledDuringPassive = false;
 }
 
 HeapVector<Member<EventTarget>> Event::pathInternal(ScriptState* scriptState, EventPathMode mode) const
 {
     if (m_target)
         HostsUsingFeatures::countHostOrIsolatedWorldHumanReadableName(scriptState, *m_target, HostsUsingFeatures::Feature::EventPath);
 
     if (!m_currentTarget) {
         DCHECK_EQ(Event::kNone, m_eventPhase);
@@ skipping 54 matching lines @@
 
 DEFINE_TRACE(Event)
 {
     visitor->trace(m_currentTarget);
     visitor->trace(m_target);
     visitor->trace(m_underlyingEvent);
     visitor->trace(m_eventPath);
 }
 
 } // namespace blink