Bug 1831580 - Post a Transform notification for instant scrolls. r=hiro

Keyboard scrolls and any other non-script driven instant scrolls handled
by APZ, should post TransformBegin and TransformEnd notifications.

Depends on D187051

Differential Revision: https://phabricator.services.mozilla.com/D188519

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -5532,6 +5532,7 @@ void AsyncPanZoomController::NotifyLayersUpdated(
     }
   }
 
+  bool instantScrollMayTriggerTransform = false;
   bool scrollOffsetUpdated = false;
   bool smoothScrollRequested = false;
   bool didCancelAnimation = false;
@@ -5609,6 +5610,11 @@ void AsyncPanZoomController::NotifyLayersUpdated(
     MOZ_ASSERT(scrollUpdate.GetMode() == ScrollMode::Instant ||
                scrollUpdate.GetMode() == ScrollMode::Normal);
 
+    instantScrollMayTriggerTransform =
+        scrollUpdate.GetMode() == ScrollMode::Instant &&
+        scrollUpdate.GetScrollTriggeredByScript() ==
+            ScrollTriggeredByScript::No;
+
     // If the layout update is of a higher priority than the visual update, then
     // we don't want to apply the visual update.
     // If the layout update is of a clobbering type (or a smooth scroll request,
@@ -5741,6 +5747,15 @@ void AsyncPanZoomController::NotifyLayersUpdated(
     // Since the main-thread scroll offset changed we should trigger a
     // recomposite to make sure it becomes user-visible.
     ScheduleComposite();
+
+    // If the scroll offset was updated, we're not in a transforming state,
+    // and we are scrolling by a non-zero delta, we should ensure
+    // TransformBegin and TransformEnd notifications are sent.
+    if (!IsTransformingState(mState) && instantScrollMayTriggerTransform &&
+        cumulativeRelativeDelta && *cumulativeRelativeDelta != CSSPoint() &&
+        !didCancelAnimation) {
+      SendTransformBeginAndEnd();
+    }
   }
 
   // If our scroll range changed (for example, because the page dynamically
@@ -6264,6 +6279,14 @@ void AsyncPanZoomController::DispatchStateChangeNotification(
     }
   }
 }
+void AsyncPanZoomController::SendTransformBeginAndEnd() {
+  RefPtr<GeckoContentController> controller = GetGeckoContentController();
+  if (controller) {
+    controller->NotifyAPZStateChange(GetGuid(),
+                                     APZStateChange::eTransformBegin);
+    controller->NotifyAPZStateChange(GetGuid(), APZStateChange::eTransformEnd);
+  }
+}
 
 bool AsyncPanZoomController::IsInTransformingState() const {
   RecursiveMutexAutoLock lock(mRecursiveMutex);

gfx/layers/apz/src/AsyncPanZoomController.h

@@ -1477,6 +1477,13 @@ class AsyncPanZoomController {
    */
   void DispatchStateChangeNotification(PanZoomState aOldState,
                                        PanZoomState aNewState);
+
+  /**
+   * Send a TransformBegin notification followed by a TransformEnd
+   * notification.
+   */
+  void SendTransformBeginAndEnd();
+
   /**
    * Internal helpers for checking general state of this apzc.
    */

gfx/layers/apz/test/mochitest/test_group_scrollend.html

@@ -67,9 +67,8 @@
         "prefs": smoothScrollEnabled},
   {"file": "helper_keyboard_scrollend.html?direction=down&flush-before-key",
         "prefs": smoothScrollDisabled},
-  // FIXME(dlrobertson): When bug 1831580 is fixed, add additional tests
-  // for scrollend events following keyboard scrolls that do change the
-  // scroll position without being handled by a APZC on-keyboard scroll.
+  {"file": "helper_keyboard_scrollend.html?direction=down",
+        "prefs": smoothScrollDisabled},
 ];
 
 if (isApzEnabled()) {