Bug 1638314 - Add some telemetry around wheel modifiers and triggered actions. r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D76044

dom/events/EventStateManager.cpp

@@ -3350,12 +3350,14 @@ nsresult EventStateManager::PostHandleEvent(nsPresContext* aPresContext,
       WidgetWheelEvent* wheelEvent = aEvent->AsWheelEvent();
       MOZ_ASSERT(wheelEvent);
 
-      // When APZ is enabled, the actual scroll animation might be handled by
-      // the compositor.
       WheelPrefs::Action action =
-          wheelEvent->mFlags.mHandledByAPZ
-              ? WheelPrefs::ACTION_NONE
-              : WheelPrefs::GetInstance()->ComputeActionFor(wheelEvent);
+          WheelPrefs::GetInstance()->RecordTelemetryAndComputeActionFor(
+              wheelEvent);
+      if (wheelEvent->mFlags.mHandledByAPZ) {
+        // When APZ is enabled, the actual scroll animation might be handled by
+        // the compositor.
+        action = WheelPrefs::ACTION_NONE;
+      }
 
       WheelDeltaAdjustmentStrategy strategy =
           GetWheelDeltaAdjustmentStrategy(*wheelEvent);
@@ -6203,6 +6205,18 @@ EventStateManager::WheelPrefs::ComputeActionFor(
   return actions[index];
 }
 
+EventStateManager::WheelPrefs::Action
+EventStateManager::WheelPrefs::RecordTelemetryAndComputeActionFor(
+    const WidgetWheelEvent* aEvent) {
+  Index index = GetIndexFor(aEvent);
+  Action action = ComputeActionFor(aEvent);
+  mozilla::Telemetry::Accumulate(mozilla::Telemetry::WHEEL_INDEX,
+                                 (uint32_t)index);
+  mozilla::Telemetry::Accumulate(mozilla::Telemetry::WHEEL_ACTION,
+                                 (uint32_t)action);
+  return action;
+}
+
 bool EventStateManager::WheelPrefs::NeedToComputeLineOrPageDelta(
     const WidgetWheelEvent* aEvent) {
   Index index = GetIndexFor(aEvent);

dom/events/EventStateManager.h

@@ -641,6 +641,13 @@ class EventStateManager : public nsSupportsWeakReference, public nsIObserver {
     };
     Action ComputeActionFor(const WidgetWheelEvent* aEvent);
 
+    /**
+     * Same as ComputeActionFor, but also records telemetry probes about the
+     * event. This is a member of WheelPrefs mostly to avoid exposing private
+     * members.
+     */
+    Action RecordTelemetryAndComputeActionFor(const WidgetWheelEvent* aEvent);
+
     /**
      * NeedToComputeLineOrPageDelta() returns if the aEvent needs to be
      * computed the lineOrPageDelta values.

toolkit/components/telemetry/Histograms.json

@@ -15209,5 +15209,27 @@
     "releaseChannelCollection": "opt-out",
     "bug_numbers": [1619349],
     "description": "Encountered a missing character that would have been supported by a user-installed font"
+  },
+  "WHEEL_INDEX" : {
+    "products": ["firefox"],
+    "record_in_processes": ["main"],
+    "alert_emails": ["kgupta@mozilla.com"],
+    "expires_in_version": "80",
+    "kind" : "enumerated",
+    "bug_numbers": [1638314],
+    "description": "Keyboard modifier key applied to wheel events (WheelPrefs::Index)",
+    "n_values": 7,
+    "releaseChannelCollection": "opt-out"
+  },
+  "WHEEL_ACTION" : {
+    "products": ["firefox"],
+    "record_in_processes": ["main"],
+    "alert_emails": ["kgupta@mozilla.com"],
+    "expires_in_version": "80",
+    "kind" : "enumerated",
+    "bug_numbers": [1638314],
+    "description": "Action triggered by wheel event (WheelPrefs::Action)",
+    "n_values": 10,
+    "releaseChannelCollection": "opt-out"
   }
 }