diff --git a/dom/base/nsContentUtils.cpp b/dom/base/nsContentUtils.cpp
index 47527ece2319..15e6b0aaba93 100644
--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -10686,6 +10686,29 @@ nsContentUtils::GetSourceMapURL(nsIHttpChannel* aChannel, nsACString& aResult)
   return NS_SUCCEEDED(rv);
 }
 
+/* static */  bool
+nsContentUtils::IsMessageInputEvent(const IPC::Message& aMsg)
+{
+  if ((aMsg.type() & mozilla::dom::PBrowser::PBrowserStart)
+      == mozilla::dom::PBrowser::PBrowserStart) {
+    switch (aMsg.type()) {
+      case mozilla::dom::PBrowser::Msg_RealMouseMoveEvent__ID:
+      case mozilla::dom::PBrowser::Msg_RealMouseButtonEvent__ID:
+      case mozilla::dom::PBrowser::Msg_RealKeyEvent__ID:
+      case mozilla::dom::PBrowser::Msg_MouseWheelEvent__ID:
+      case mozilla::dom::PBrowser::Msg_RealTouchEvent__ID:
+      case mozilla::dom::PBrowser::Msg_RealTouchMoveEvent__ID:
+      case mozilla::dom::PBrowser::Msg_RealDragEvent__ID:
+      case mozilla::dom::PBrowser::Msg_UpdateDimensions__ID:
+      case mozilla::dom::PBrowser::Msg_MouseEvent__ID:
+      case mozilla::dom::PBrowser::Msg_KeyEvent__ID:
+      case mozilla::dom::PBrowser::Msg_SetDocShellIsActive__ID:
+        return true;
+    }
+  }
+  return false;
+}
+
 static const char* kUserInteractionInactive = "user-interaction-inactive";
 static const char* kUserInteractionActive = "user-interaction-active";
 
diff --git a/dom/base/nsContentUtils.h b/dom/base/nsContentUtils.h
index 79084ff586a6..e29a181987ff 100644
--- a/dom/base/nsContentUtils.h
+++ b/dom/base/nsContentUtils.h
@@ -3126,6 +3126,13 @@ public:
    */
   static bool GetSourceMapURL(nsIHttpChannel* aChannel, nsACString& aResult);
 
+  /**
+   * Returns true if the passed-in mesasge is a pending InputEvent.
+   *
+   * @param aMsg  The message to check
+   */
+  static bool IsMessageInputEvent(const IPC::Message& aMsg);
+
 private:
   static bool InitializeEventTable();
 
diff --git a/dom/ipc/ContentChild.cpp b/dom/ipc/ContentChild.cpp
index 2bbf599922a5..6dfc0a8680bf 100644
--- a/dom/ipc/ContentChild.cpp
+++ b/dom/ipc/ContentChild.cpp
@@ -491,6 +491,29 @@ private:
   }
 };
 
+#ifdef NIGHTLY_BUILD
+/**
+ * The singleton of this class is registered with the HangMonitor as an
+ * annotator, so that the hang monitor can record whether or not there were
+ * pending input events when the thread hung.
+ */
+class PendingInputEventHangAnnotator final
+  : public HangMonitor::Annotator
+{
+public:
+  virtual void AnnotateHang(HangMonitor::HangAnnotations& aAnnotations)
+  {
+    int32_t pending = ContentChild::GetSingleton()->GetPendingInputEvents();
+    if (pending > 0) {
+      aAnnotations.AddAnnotation(NS_LITERAL_STRING("PendingInput"), pending);
+    }
+  }
+
+  static PendingInputEventHangAnnotator sSingleton;
+};
+PendingInputEventHangAnnotator PendingInputEventHangAnnotator::sSingleton;
+#endif
+
 NS_IMPL_ISUPPORTS(BackgroundChildPrimer, nsIIPCBackgroundChildCreateCallback)
 
 ContentChild* ContentChild::sSingleton;
@@ -638,6 +661,10 @@ ContentChild::Init(MessageLoop* aIOLoop,
 
   SetProcessName(NS_LITERAL_STRING("Web Content"));
 
+#ifdef NIGHTLY_BUILD
+  HangMonitor::RegisterAnnotator(PendingInputEventHangAnnotator::sSingleton);
+#endif
+
   return true;
 }
 
@@ -2930,6 +2957,10 @@ ContentChild::RecvShutdown()
 
   mShuttingDown = true;
 
+#ifdef NIGHTLY_BUILD
+  HangMonitor::UnregisterAnnotator(PendingInputEventHangAnnotator::sSingleton);
+#endif
+
   if (mPolicy) {
     mPolicy->Deactivate();
     mPolicy = nullptr;
@@ -3580,6 +3611,33 @@ ContentChild::GetSpecificMessageEventTarget(const Message& aMsg)
   return nullptr;
 }
 
+#ifdef NIGHTLY_BUILD
+void
+ContentChild::OnChannelReceivedMessage(const Message& aMsg)
+{
+  if (nsContentUtils::IsMessageInputEvent(aMsg)) {
+    mPendingInputEvents++;
+  }
+}
+
+PContentChild::Result
+ContentChild::OnMessageReceived(const Message& aMsg)
+{
+  if (nsContentUtils::IsMessageInputEvent(aMsg)) {
+    DebugOnly<uint32_t> prevEvts = mPendingInputEvents--;
+    MOZ_ASSERT(prevEvts > 0);
+  }
+
+  return PContentChild::OnMessageReceived(aMsg);
+}
+
+PContentChild::Result
+ContentChild::OnMessageReceived(const Message& aMsg, Message*& aReply)
+{
+  return PContentChild::OnMessageReceived(aMsg, aReply);
+}
+#endif
+
 } // namespace dom
 
 #if !defined(XP_WIN)
diff --git a/dom/ipc/ContentChild.h b/dom/ipc/ContentChild.h
index 73d41c783804..2f6f36640741 100644
--- a/dom/ipc/ContentChild.h
+++ b/dom/ipc/ContentChild.h
@@ -681,6 +681,17 @@ public:
                     nsTArray<PluginTag>&& aPluginTags,
                     nsTArray<FakePluginTag>&& aFakePluginTags) override;
 
+#ifdef NIGHTLY_BUILD
+  // Fetch the current number of pending input events.
+  //
+  // NOTE: This method performs an atomic read, and is safe to call from all threads.
+  uint32_t
+  GetPendingInputEvents()
+  {
+    return mPendingInputEvents;
+  }
+#endif
+
 private:
   static void ForceKillTimerCallback(nsITimer* aTimer, void* aClosure);
   void StartForceKillTimer();
@@ -695,6 +706,17 @@ private:
   virtual already_AddRefed<nsIEventTarget>
   GetSpecificMessageEventTarget(const Message& aMsg) override;
 
+#ifdef NIGHTLY_BUILD
+  virtual void
+  OnChannelReceivedMessage(const Message& aMsg) override;
+
+  virtual PContentChild::Result
+  OnMessageReceived(const Message& aMsg) override;
+
+  virtual PContentChild::Result
+  OnMessageReceived(const Message& aMsg, Message*& aReply) override;
+#endif
+
   InfallibleTArray<nsAutoPtr<AlertObserver> > mAlertObservers;
   RefPtr<ConsoleListener> mConsoleListener;
 
@@ -765,6 +787,11 @@ private:
 
   mozilla::Atomic<bool> mShuttingDown;
 
+#ifdef NIGHTLY_BUILD
+  // NOTE: This member is atomic because it can be accessed from off-main-thread.
+  mozilla::Atomic<uint32_t> mPendingInputEvents;
+#endif
+
   DISALLOW_EVIL_CONSTRUCTORS(ContentChild);
 };
 
diff --git a/widget/PuppetWidget.cpp b/widget/PuppetWidget.cpp
index bc0fe77e5cc6..defb358e2656 100644
--- a/widget/PuppetWidget.cpp
+++ b/widget/PuppetWidget.cpp
@@ -1404,23 +1404,9 @@ PuppetWidget::HasPendingInputEvent()
 
   mTabChild->GetIPCChannel()->PeekMessages(
     [&ret](const IPC::Message& aMsg) -> bool {
-      if ((aMsg.type() & mozilla::dom::PBrowser::PBrowserStart)
-          == mozilla::dom::PBrowser::PBrowserStart) {
-        switch (aMsg.type()) {
-          case mozilla::dom::PBrowser::Msg_RealMouseMoveEvent__ID:
-          case mozilla::dom::PBrowser::Msg_RealMouseButtonEvent__ID:
-          case mozilla::dom::PBrowser::Msg_RealKeyEvent__ID:
-          case mozilla::dom::PBrowser::Msg_MouseWheelEvent__ID:
-          case mozilla::dom::PBrowser::Msg_RealTouchEvent__ID:
-          case mozilla::dom::PBrowser::Msg_RealTouchMoveEvent__ID:
-          case mozilla::dom::PBrowser::Msg_RealDragEvent__ID:
-          case mozilla::dom::PBrowser::Msg_UpdateDimensions__ID:
-          case mozilla::dom::PBrowser::Msg_MouseEvent__ID:
-          case mozilla::dom::PBrowser::Msg_KeyEvent__ID:
-          case mozilla::dom::PBrowser::Msg_SetDocShellIsActive__ID:
-            ret = true;
-            return false;  // Stop peeking.
-        }
+      if (nsContentUtils::IsMessageInputEvent(aMsg)) {
+        ret = true;
+        return false; // Stop peeking.
       }
       return true;
     }