Bug 1630781 - Eliminate the frame_delay pref, assume it true everywhere. r=botond,mstange

Having to think about multiple codepaths adds complexity and it doesn't seem
like we're going to turn this pref back off anytime soon.

Differential Revision: https://phabricator.services.mozilla.com/D72040

gfx/layers/apz/src/APZCTreeManager.cpp

@@ -891,7 +891,7 @@ void APZCTreeManager::SampleForWebRender(
   // Advance animations. It's important that this happens after
   // sampling all async transforms, because AdvanceAnimations() updates
   // the effective scroll offset to the value it should have for the *next*
-  // composite after this one (if the APZ frame delay is enabled).
+  // composite after this one.
   bool activeAnimations =
       AdvanceAnimationsInternal(lock, Some(aRenderRoot), aSampleTime);
   if (activeAnimations) {

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -321,13 +321,6 @@ typedef PlatformSpecificStateBase
  * NOTE: Should not be set to anything
  * other than 0.0 for Android except for tests to disable flings.
  *
- * \li\b apz.frame_delay.enabled
- * If this is set to true, changes to the async scroll offset and async zoom
- * will not be immediately reflected in GetCurrentAsyncTransform() when called
- * with |AsyncTransformConsumer::eForCompositing|. Rather, the transform will
- * reflect the value of the async scroll offset and async zoom at the last time
- * SampleCompositedAsyncTransform() was called.
- *
  * \li\b apz.keyboard.enabled
  * Determines whether scrolling with the keyboard will be allowed to be handled
  * by APZ.
@@ -3156,12 +3149,7 @@ void AsyncPanZoomController::UpdateWithTouchAtDevicePoint(
 }
 
 Maybe<CompositionPayload> AsyncPanZoomController::NotifyScrollSampling() {
-  if (StaticPrefs::apz_frame_delay_enabled()) {
-    return std::move(mCompositedScrollPayload);
-  }
-  // If frame.delay disabled, the triggering events are those
-  // from the most recent frame
-  return std::move(mScrollPayload);
+  return std::move(mCompositedScrollPayload);
 }
 
 bool AsyncPanZoomController::AttemptScroll(
@@ -4187,7 +4175,7 @@ CSSRect AsyncPanZoomController::GetEffectiveLayoutViewport(
   if (aMode == eForCompositing && mScrollMetadata.IsApzForceDisabled()) {
     return mLastContentPaintMetrics.GetLayoutViewport();
   }
-  if (aMode == eForCompositing && StaticPrefs::apz_frame_delay_enabled()) {
+  if (aMode == eForCompositing) {
     return mCompositedLayoutViewport;
   }
   return Metrics().GetLayoutViewport();
@@ -4198,7 +4186,7 @@ CSSPoint AsyncPanZoomController::GetEffectiveScrollOffset(
   if (aMode == eForCompositing && mScrollMetadata.IsApzForceDisabled()) {
     return mLastContentPaintMetrics.GetVisualViewportOffset();
   }
-  if (aMode == eForCompositing && StaticPrefs::apz_frame_delay_enabled()) {
+  if (aMode == eForCompositing) {
     return mCompositedScrollOffset;
   }
   return Metrics().GetScrollOffset();
@@ -4209,7 +4197,7 @@ CSSToParentLayerScale2D AsyncPanZoomController::GetEffectiveZoom(
   if (aMode == eForCompositing && mScrollMetadata.IsApzForceDisabled()) {
     return mLastContentPaintMetrics.GetZoom();
   }
-  if (aMode == eForCompositing && StaticPrefs::apz_frame_delay_enabled()) {
+  if (aMode == eForCompositing) {
     return mCompositedZoom;
   }
   return Metrics().GetZoom();
@@ -4223,9 +4211,6 @@ bool AsyncPanZoomController::SampleCompositedAsyncTransform(
     mCompositedLayoutViewport = Metrics().GetLayoutViewport();
     mCompositedScrollOffset = Metrics().GetScrollOffset();
     mCompositedZoom = Metrics().GetZoom();
-
-    // If frame.delay enabled, we pass along the scroll events from
-    // frame before since they were what moved the items on the layer
     mCompositedScrollPayload = std::move(mScrollPayload);
     return true;
   }

gfx/layers/apz/src/AsyncPanZoomController.h

@@ -1165,10 +1165,6 @@ class AsyncPanZoomController {
    *
    * Returns true if the newly sampled value is different from the previously
    * sampled value.
-   *
-   * (This is only relevant when StaticPrefs::apz_frame_delay_enabled() is
-   * true. Otherwise, GetCurrentAsyncTransform() always reflects what's stored
-   * in |Metrics()| immediately, without any delay.)
    */
   bool SampleCompositedAsyncTransform(
       const RecursiveMutexAutoLock& aProofOfLock);

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

@@ -58,9 +58,7 @@ function* test(testDriver) {
   // (for unrelated reasons), and the repaint will clear the checkerboard
   // state. We do, however, want to wait for a "steady state" here that
   // includes all pending paints from the main thread and a composite that
-  // samples the APZ state. (The latter is not required if the APZ frame delay
-  // is disabled, but it's enabled by default so it's desirable to handle that
-  // situation as well.) In order to accomplish this we wait for all the main
+  // samples the APZ state. In order to accomplish this we wait for all the main
   // thread paints, and then force a composite via advanceTimeAndRefresh. The
   // advanceTimeAndRefresh has the additional advantage of freezing the refresh
   // driver which avoids any additional externally-triggered repaints from

modules/libpref/init/StaticPrefList.yaml

@@ -407,11 +407,6 @@
   value: 0.01f
   mirror: always
 
-- name: apz.frame_delay.enabled
-  type: RelaxedAtomicBool
-  value: true
-  mirror: always
-
 - name: apz.touch_acceleration_factor_x
   type: float
   value: 1.0f