Bug 1670003 - Move some of the SCROLL_INPUT_METHODS recording. r=botond

In particular, for "continuous" scrolling methods - autoscroll, scrollbar
dragging, trackpad panning, and touch panning, this patch records the *start*
of the gesture rather than on every frame or input event *during* the gesture.
This conceptually changes the metric from being implementation-centric to being
more user-centric, in that each conceptual "user gesture" counts towards one
SCROLL_INPUT_METHOD record. This is more useful for the purposes of recording
user intent with respect to scroll input methods for experiments.

Depends on D92995

Differential Revision: https://phabricator.services.mozilla.com/D92996

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -931,6 +931,10 @@ nsEventStatus AsyncPanZoomController::HandleDragEvent(
   if (aEvent.mType == MouseInput::MouseType::MOUSE_DOWN) {
     APZC_LOG("%p starting scrollbar drag\n", this);
     SetState(SCROLLBAR_DRAG);
+
+    mozilla::Telemetry::Accumulate(
+        mozilla::Telemetry::SCROLL_INPUT_METHODS,
+        (uint32_t)ScrollInputMethod::ApzScrollbarDrag);
   }
 
   if (aEvent.mType != MouseInput::MouseType::MOUSE_MOVE) {
@@ -944,9 +948,6 @@ nsEventStatus AsyncPanZoomController::HandleDragEvent(
   MOZ_ASSERT(scrollbarData.mDirection.isSome());
   ScrollDirection direction = *scrollbarData.mDirection;
 
-  mozilla::Telemetry::Accumulate(mozilla::Telemetry::SCROLL_INPUT_METHODS,
-                                 (uint32_t)ScrollInputMethod::ApzScrollbarDrag);
-
   bool isMouseAwayFromThumb = false;
   if (int snapMultiplier = StaticPrefs::slider_snapMultiplier_AtStartup()) {
     // It's fine to ignore the async component of the thumb's transform,
@@ -1207,6 +1208,9 @@ void AsyncPanZoomController::StartAutoscroll(const ScreenPoint& aPoint) {
   // Cancel any existing animation.
   CancelAnimation();
 
+  Telemetry::Accumulate(Telemetry::SCROLL_INPUT_METHODS,
+                        (uint32_t)ScrollInputMethod::ApzAutoscrolling);
+
   SetState(AUTOSCROLL);
   StartAnimation(new AutoscrollAnimation(*this, aPoint));
 }
@@ -2441,6 +2445,9 @@ nsEventStatus AsyncPanZoomController::OnPanBegin(
 
   StartTouch(aEvent.mLocalPanStartPoint, aEvent.mTimeStamp);
 
+  mozilla::Telemetry::Accumulate(mozilla::Telemetry::SCROLL_INPUT_METHODS,
+                                 (uint32_t)ScrollInputMethod::ApzPanGesture);
+
   if (GetAxisLockMode() == FREE) {
     SetState(PANNING);
     return nsEventStatus_eConsumeNoDefault;
@@ -2573,9 +2580,6 @@ nsEventStatus AsyncPanZoomController::OnPan(const PanGestureInput& aEvent,
 
   HandlePanningUpdate(physicalPanDisplacement);
 
-  mozilla::Telemetry::Accumulate(mozilla::Telemetry::SCROLL_INPUT_METHODS,
-                                 (uint32_t)ScrollInputMethod::ApzPanGesture);
-
   ScreenPoint panDistance(fabs(physicalPanDisplacement.x),
                           fabs(physicalPanDisplacement.y));
   OverscrollHandoffState handoffState(
@@ -3105,6 +3109,9 @@ nsEventStatus AsyncPanZoomController::StartPanning(
   }
 
   if (IsInPanningState()) {
+    mozilla::Telemetry::Accumulate(mozilla::Telemetry::SCROLL_INPUT_METHODS,
+                                   (uint32_t)ScrollInputMethod::ApzTouch);
+
     if (RefPtr<GeckoContentController> controller =
             GetGeckoContentController()) {
       controller->NotifyAPZStateChange(GetGuid(),
@@ -3563,8 +3570,6 @@ void AsyncPanZoomController::TrackTouch(const MultiTouchInput& aEvent) {
 
   UpdateWithTouchAtDevicePoint(aEvent);
   if (prevTouchPoint != touchPoint) {
-    mozilla::Telemetry::Accumulate(mozilla::Telemetry::SCROLL_INPUT_METHODS,
-                                   (uint32_t)ScrollInputMethod::ApzTouch);
     MOZ_ASSERT(GetCurrentTouchBlock());
     OverscrollHandoffState handoffState(
         *GetCurrentTouchBlock()->GetOverscrollHandoffChain(), panVector,

gfx/layers/apz/src/AutoscrollAnimation.cpp

@@ -12,7 +12,6 @@
 #include "APZCTreeManager.h"
 #include "FrameMetrics.h"
 #include "mozilla/Telemetry.h"  // for Telemetry
-#include "mozilla/layers/ScrollInputMethods.h"  // for ScrollInputMethod
 
 namespace mozilla {
 namespace layers {
@@ -42,9 +41,6 @@ bool AutoscrollAnimation::DoSample(FrameMetrics& aFrameMetrics,
     return false;
   }
 
-  Telemetry::Accumulate(Telemetry::SCROLL_INPUT_METHODS,
-                        (uint32_t)ScrollInputMethod::ApzAutoscrolling);
-
   ScreenPoint mouseLocation = treeManager->GetCurrentMousePosition();
 
   // The implementation of this function closely mirrors that of its main-

layout/xul/nsSliderFrame.cpp

@@ -523,10 +523,6 @@ nsresult nsSliderFrame::HandleEvent(nsPresContext* aPresContext,
           return NS_OK;
         }
 
-        mozilla::Telemetry::Accumulate(
-            mozilla::Telemetry::SCROLL_INPUT_METHODS,
-            (uint32_t)ScrollInputMethod::MainThreadScrollbarDrag);
-
         // take our current position and subtract the start location
         pos -= mDragStart;
         bool isMouseOutsideThumb = false;
@@ -1121,6 +1117,19 @@ nsresult nsSliderFrame::StopDrag() {
   AddListener();
   DragThumb(false);
 
+  if (!mScrollingWithAPZ) {
+    // We record this one at the end of the drag rather than at the beginning
+    // because at the point that the main thread starts the drag (in StartDrag)
+    // it may not know for sure whether APZ or the main thread will end up
+    // handling the drag. Even if mScrollingWithAPZ is true initially, it
+    // may get set to false if APZ rejects the drag. But by the end of the drag
+    // the mScrollingWithAPZ flag should be correct and so we can use it here
+    // to determine if APZ or the main thread handled the drag.
+    mozilla::Telemetry::Accumulate(
+        mozilla::Telemetry::SCROLL_INPUT_METHODS,
+        (uint32_t)ScrollInputMethod::MainThreadScrollbarDrag);
+  }
+
   mScrollingWithAPZ = false;
 
   UnsuppressDisplayport();

toolkit/actors/AutoScrollChild.jsm

@@ -235,6 +235,11 @@ class AutoScrollChild extends JSWindowActorChild {
     let content = this.document.defaultView;
     this._lastFrame = content.performance.now();
     content.requestAnimationFrame(this.autoscrollLoop);
+
+    const kAutoscroll = 15; // defined in mozilla/layers/ScrollInputMethods.h
+    Services.telemetry
+      .getHistogramById("SCROLL_INPUT_METHODS")
+      .add(kAutoscroll);
   }
 
   stopScroll() {
@@ -309,11 +314,6 @@ class AutoScrollChild extends JSWindowActorChild {
       this._scrollErrorX = desiredScrollX - actualScrollX;
     }
 
-    const kAutoscroll = 15; // defined in mozilla/layers/ScrollInputMethods.h
-    Services.telemetry
-      .getHistogramById("SCROLL_INPUT_METHODS")
-      .add(kAutoscroll);
-
     this._scrollable.scrollBy({
       left: actualScrollX,
       top: actualScrollY,