Bug 1887515 - Remove the apz.drag.initial.enabled pref. r=botond

Differential Revision: https://phabricator.services.mozilla.com/D205544

gfx/layers/apz/src/APZCTreeManager.cpp

@@ -2290,8 +2290,7 @@ void APZCTreeManager::SetupScrollbarDrag(
 
   // Under some conditions, we can confirm the drag block right away.
   // Otherwise, we have to wait for a main-thread confirmation.
-  if (StaticPrefs::apz_drag_initial_enabled() &&
-      // check that the scrollbar's target scroll frame is layerized
+  if (/* check that the scrollbar's target scroll frame is layerized */
       aScrollThumbNode->GetScrollTargetId() == aApzc->GetGuid().mScrollId &&
       !aApzc->IsScrollInfoLayer()) {
     uint64_t dragBlockId = dragBlock->GetBlockId();

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -242,11 +242,6 @@ typedef PlatformSpecificStateBase
  * Setting this pref to true will cause APZ to handle mouse-dragging of
  * scrollbar thumbs.
  *
- * \li\b apz.drag.initial.enabled
- * Setting this pref to true will cause APZ to try to handle mouse-dragging
- * of scrollbar thumbs without an initial round-trip to content to start it
- * if possible. Only has an effect if apz.drag.enabled is also true.
- *
  * \li\b apz.drag.touch.enabled
  * Setting this pref to true will cause APZ to handle touch-dragging of
  * scrollbar thumbs. Only has an effect if apz.drag.enabled is also true.

modules/libpref/init/StaticPrefList.yaml

@@ -478,11 +478,6 @@
   value: true
   mirror: always
 
-- name: apz.drag.initial.enabled
-  type: RelaxedAtomicBool
-  value: true
-  mirror: always
-
 - name: apz.drag.touch.enabled
   type: RelaxedAtomicBool
   value: true