Bug 1630781 - Allow mSampledState to temporarily hold more than one state. r=botond

Again, functionally this is a no-op since instead of replacing the back()
element, we do an emplace_back() followed by a pop_front().

Differential Revision: https://phabricator.services.mozilla.com/D72044

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -3984,6 +3984,7 @@ bool AsyncPanZoomController::UpdateAnimation(
   // call this after the |mLastSampleTime == aSampleTime| check, to ensure
   // it's only called once per APZC on each composite.
   bool needComposite = SampleCompositedAsyncTransform(aProofOfLock);
+  AdvanceToNextSample();
 
   TimeDuration sampleTimeDelta = aSampleTime - mLastSampleTime;
   mLastSampleTime = aSampleTime;
@@ -4199,15 +4200,29 @@ CSSToParentLayerScale2D AsyncPanZoomController::GetEffectiveZoom(
   return Metrics().GetZoom();
 }
 
+void AsyncPanZoomController::AdvanceToNextSample() {
+  AssertOnSamplerThread();
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  // Always keep at least one state in mSampledState.
+  if (mSampledState.size() > 1) {
+    mSampledState.pop_front();
+  }
+}
+
 bool AsyncPanZoomController::SampleCompositedAsyncTransform(
     const RecursiveMutexAutoLock& aProofOfLock) {
   MOZ_ASSERT(mSampledState.size() == 1);
-  if (mSampledState.back() != SampledAPZCState(Metrics())) {
-    mSampledState.back() =
-        SampledAPZCState(Metrics(), std::move(mScrollPayload));
-    return true;
-  }
-  return false;
+  bool sampleChanged = (mSampledState.back() != SampledAPZCState(Metrics()));
+  mSampledState.emplace_back(Metrics(), std::move(mScrollPayload));
+  return sampleChanged;
+}
+
+void AsyncPanZoomController::ResampleCompositedAsyncTransform(
+    const RecursiveMutexAutoLock& aProofOfLock) {
+  // This only gets called during testing situations, so the fact that this
+  // drops the scroll payload from mSampledState.front() is not really a
+  // problem.
+  mSampledState.front() = SampledAPZCState(Metrics());
 }
 
 void AsyncPanZoomController::ApplyAsyncTestAttributes(
@@ -4215,9 +4230,16 @@ void AsyncPanZoomController::ApplyAsyncTestAttributes(
   if (mTestAttributeAppliers == 0) {
     if (mTestAsyncScrollOffset != CSSPoint() ||
         mTestAsyncZoom != LayerToParentLayerScale()) {
+      // TODO Currently we update Metrics() and resample, which will cause
+      // the very latest user input to get immediately captured in the sample,
+      // and may defeat our attempt at "frame delay" (i.e. delaying the user
+      // input from affecting composition by one frame).
+      // Instead, maybe we should just apply the mTest* stuff directly to
+      // mSampledState.front(). We can even save/restore that SampledAPZCState
+      // instance in the AutoApplyAsyncTestAttributes instead of Metrics().
       Metrics().ZoomBy(mTestAsyncZoom.scale);
       ScrollBy(mTestAsyncScrollOffset);
-      SampleCompositedAsyncTransform(aProofOfLock);
+      ResampleCompositedAsyncTransform(aProofOfLock);
     }
   }
   ++mTestAttributeAppliers;
@@ -4232,7 +4254,7 @@ void AsyncPanZoomController::UnapplyAsyncTestAttributes(
     if (mTestAsyncScrollOffset != CSSPoint() ||
         mTestAsyncZoom != LayerToParentLayerScale()) {
       Metrics() = aPrevFrameMetrics;
-      SampleCompositedAsyncTransform(aProofOfLock);
+      ResampleCompositedAsyncTransform(aProofOfLock);
     }
   }
 }

gfx/layers/apz/src/AsyncPanZoomController.h

@@ -961,7 +961,7 @@ class AsyncPanZoomController {
   FrameMetrics mExpectedGeckoMetrics;
 
   // This holds important state from the Metrics() at previous times
-  // SampleCompositedAsyncTransform() was called. This will always have exactly
+  // SampleCompositedAsyncTransform() was called. This will always have at least
   // one item. mRecursiveMutex must be held when using or modifying this member.
   // Samples should be inserted to the "back" of the deque and extracted from
   // the "front".
@@ -1143,16 +1143,35 @@ class AsyncPanZoomController {
 
  private:
   /**
-   * Samples the composited async transform, making the result of
+   * Advances to the next sample, if there is one, the list of sampled states
+   * stored in mSampledState. This will make the result of
    * |GetCurrentAsyncTransform(eForCompositing)| and similar functions reflect
-   * the async scroll offset and zoom stored in |Metrics()|.
+   * the async scroll offset and zoom of the next sample. See also
+   * SampleCompositedAsyncTransform which creates the samples.
+   */
+  void AdvanceToNextSample();
+
+  /**
+   * Samples the composited async transform, storing the result into
+   * mSampledState. This will make the result of
+   * |GetCurrentAsyncTransform(eForCompositing)| and similar functions reflect
+   * the async scroll offset and zoom stored in |Metrics()| when the sample
+   * is activated via some future call to |AdvanceToNextSample|.
    *
-   * Returns true if the newly sampled value is different from the previously
+   * Returns true if the newly sampled value is different from the last
    * sampled value.
    */
   bool SampleCompositedAsyncTransform(
       const RecursiveMutexAutoLock& aProofOfLock);
 
+  /**
+   * Updates the sample at the front of mSampledState with the latest
+   * metrics. This makes the result of
+   * |GetCurrentAsyncTransform(eForCompositing)| reflect the current Metrics().
+   */
+  void ResampleCompositedAsyncTransform(
+      const RecursiveMutexAutoLock& aProofOfLock);
+
   /*
    * Helper functions to query the async layout viewport, scroll offset, and
    * zoom either directly from |Metrics()|, or from cached variables that