Bug 1219150 - EventStateManager::{LatestUserInputStart, UserInputCount};r=smaug

To detect slow behaviors, it is useful to find out whether a user
input was handled between two instants.

This patch introduces EventStateManager::UserInputCount, which lets
client code determine whether user input was handled between two
points of execution, and EventStateManager::LatestUserInputStart,
which lets client code find out when the altest user input was
handled.

dom/events/EventStateManager.cpp

@@ -264,6 +264,7 @@ NS_INTERFACE_MAP_END
 static uint32_t sESMInstanceCount = 0;
 static bool sPointerEventEnabled = false;
 
+uint64_t EventStateManager::sUserInputCounter = 0;
 int32_t EventStateManager::sUserInputEventDepth = 0;
 bool EventStateManager::sNormalLMouseEventInProcess = false;
 EventStateManager* EventStateManager::sActiveESM = nullptr;
@@ -279,6 +280,7 @@ nsWeakPtr EventStateManager::sPointerLockedElement;
 // Reference to the document which requested pointer lock.
 nsWeakPtr EventStateManager::sPointerLockedDoc;
 nsCOMPtr<nsIContent> EventStateManager::sDragOverContent = nullptr;
+TimeStamp EventStateManager::sLatestUserInputStart;
 TimeStamp EventStateManager::sHandlingInputStart;
 
 EventStateManager::WheelPrefs*

dom/events/EventStateManager.h

@@ -195,8 +195,9 @@ public:
   static void StartHandlingUserInput()
   {
     ++sUserInputEventDepth;
+    ++sUserInputCounter;
     if (sUserInputEventDepth == 1) {
-      sHandlingInputStart = TimeStamp::Now();
+      sLatestUserInputStart = sHandlingInputStart = TimeStamp::Now();
     }
   }
 
@@ -209,17 +210,38 @@ public:
   }
 
   /**
-   * Returns true if the current code is being executed as a result of user input.
-   * This includes timers or anything else that is initiated from user input.
-   * However, mouse over events are not counted as user input, nor are
-   * page load events. If this method is called from asynchronously executed code,
-   * such as during layout reflows, it will return false. If more time has elapsed
-   * since the user input than is specified by the
-   * dom.event.handling-user-input-time-limit pref (default 1 second), this
-   * function also returns false.
+   * Returns true if the current code is being executed as a result of
+   * user input.  This includes anything that is initiated by user,
+   * with the exception of page load events or mouse over events. If
+   * this method is called from asynchronously executed code, such as
+   * during layout reflows, it will return false. If more time has
+   * elapsed since the user input than is specified by the
+   * dom.event.handling-user-input-time-limit pref (default 1 second),
+   * this function also returns false.
    */
   static bool IsHandlingUserInput();
 
+  /**
+   * Get the number of user inputs handled since process start. This
+   * includes anything that is initiated by user, with the exception
+   * of page load events or mouse over events.
+   */
+  static uint64_t UserInputCount()
+  {
+    return sUserInputCounter;
+  }
+
+  /**
+   * Get the timestamp at which the latest user input was handled.
+   *
+   * Guaranteed to be monotonic. Until the first user input, return
+   * the epoch.
+   */
+  static TimeStamp LatestUserInputStart()
+  {
+    return sLatestUserInputStart;
+  }
+
   nsPresContext* GetPresContext() { return mPresContext; }
 
   NS_DECL_CYCLE_COLLECTION_CLASS_AMBIGUOUS(EventStateManager,
@@ -921,9 +943,14 @@ private:
 
   bool m_haveShutdown;
 
-  // Time at which we began handling user input.
+  // Time at which we began handling user input. Reset to the epoch
+  // once we have finished handling user input.
   static TimeStamp sHandlingInputStart;
 
+  // Time at which we began handling the latest user input. Not reset
+  // at the end of the input.
+  static TimeStamp sLatestUserInputStart;
+
   RefPtr<OverOutElementsWrapper> mMouseEnterLeaveHelper;
   nsRefPtrHashtable<nsUint32HashKey, OverOutElementsWrapper> mPointersEnterLeaveHelper;
 
@@ -932,6 +959,16 @@ public:
   // Array for accesskey support
   nsCOMArray<nsIContent> mAccessKeys;
 
+  // The number of user inputs handled since process start. This
+  // includes anything that is initiated by user, with the exception
+  // of page load events or mouse over events.
+  static uint64_t sUserInputCounter;
+
+  // The current depth of user inputs. This includes anything that is
+  // initiated by user, with the exception of page load events or
+  // mouse over events. Incremented whenever we start handling a user
+  // input, decremented when we have finished handling a user
+  // input. This depth is *not* reset in case of nested event loops.
   static int32_t sUserInputEventDepth;
   
   static bool sNormalLMouseEventInProcess;