Bug 1458653 - Don't accelerate a fling if the previous fling animation has slowed down below a velocity threshold. r=botond

This should prevent most cases of unintended fling acceleration. For example,
when scrolling paragraph-by-paragraph via a series of short flings, we would
often accelerate the next fling even if the previous fling had already slowed
down almost to a stop. With this change, we will no longer trigger acceleration
in that scenario.

This also matches Chrome's behavior.

This patch does not add a new pref for the new velocity threshold.
Chrome also shares the same velocity threshold for both velocities.

Differential Revision: https://phabricator.services.mozilla.com/D95469

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -259,8 +259,9 @@ typedef PlatformSpecificStateBase
  * Units: milliseconds
  *
  * \li\b apz.fling_accel_min_velocity
- * The minimum velocity of the second fling for it to be considered for fling
- * acceleration.
+ * The minimum velocity of the second fling, and the minimum velocity of the
+ * previous fling animation at the point of interruption, for the new fling to
+ * be considered for fling acceleration.
  * Units: screen pixels per milliseconds
  *
  * \li\b apz.fling_accel_base_mult

gfx/layers/apz/src/FlingAccelerator.cpp

@@ -16,6 +16,7 @@ namespace layers {
 void FlingAccelerator::Reset() {
   mPreviousFlingStartTime = SampleTime{};
   mPreviousFlingStartingVelocity = ParentLayerPoint{};
+  mPreviousFlingCancelVelocity = ParentLayerPoint{};
 }
 
 static bool SameDirection(float aVelocity1, float aVelocity2) {
@@ -53,6 +54,8 @@ ParentLayerPoint FlingAccelerator::GetFlingStartingVelocity(
     }
   }
 
+  Reset();
+
   mPreviousFlingStartTime = aNow;
   mPreviousFlingStartingVelocity = velocity;
 
@@ -81,6 +84,15 @@ bool FlingAccelerator::ShouldAccelerate(
     return false;
   }
 
+  if (mPreviousFlingCancelVelocity.Length() <
+      StaticPrefs::apz_fling_accel_min_velocity()) {
+    FLING_LOG(
+        "%p The previous fling animation had slowed down too much when it was "
+        "interrupted (%f), not accelerating.\n",
+        this, float(mPreviousFlingCancelVelocity.Length()));
+    return false;
+  }
+
   return true;
 }
 

gfx/layers/apz/src/FlingAccelerator.h

@@ -32,6 +32,10 @@ class FlingAccelerator final {
   ParentLayerPoint GetFlingStartingVelocity(const SampleTime& aNow,
                                             const ParentLayerPoint& aVelocity);
 
+  void ObserveFlingCanceled(const ParentLayerPoint& aVelocity) {
+    mPreviousFlingCancelVelocity = aVelocity;
+  }
+
  protected:
   bool ShouldAccelerate(const SampleTime& aNow,
                         const ParentLayerPoint& aVelocity) const;
@@ -41,6 +45,9 @@ class FlingAccelerator final {
   // The time at which the most recent fling started. This is the time when the
   // finger is lifted at the end of the gesture, as the fling animation starts.
   SampleTime mPreviousFlingStartTime;
+  // The velocity that the previous fling animation had at the point it was
+  // interrupted.
+  ParentLayerPoint mPreviousFlingCancelVelocity;
 };
 
 }  // namespace layers

gfx/layers/apz/src/GenericFlingAnimation.h

@@ -183,6 +183,10 @@ class GenericFlingAnimation : public AsyncPanZoomAnimation,
     return true;
   }
 
+  void Cancel(CancelAnimationFlags aFlags) override {
+    mApzc.mFlingAccelerator.ObserveFlingCanceled(mApzc.GetVelocityVector());
+  }
+
   virtual bool HandleScrollOffsetUpdate(
       const Maybe<CSSPoint>& aRelativeDelta) override {
     return true;

gfx/layers/apz/test/gtest/TestFlingAcceleration.cpp

@@ -146,3 +146,17 @@ TEST_F(APZCFlingAccelerationTester,
                          {0, 0, 14, 61, 41, 0, 45, 35});
   CHECK_VELOCITY(Up, 3.2, 4.2);
 }
+
+TEST_F(APZCFlingAccelerationTester,
+       ShouldNotAccelerateWhenPreviousFlingHasSlowedDown) {
+  SCOPED_GFX_PREF_INT("apz.fling_accel_interval_ms", 750);
+
+  ExecutePanGesture100Hz(ScreenIntPoint{748, 1046},
+                         {0, 9, 15, 23, 31, 30, 0, 34, 31, 29, 28, 24, 24, 11});
+  CHECK_VELOCITY(Up, 2.2, 3.0);
+  ExecuteWait(TimeDuration::FromMilliseconds(498));
+  CHECK_VELOCITY(Up, 0.8, 1.0);
+  ExecutePanGesture100Hz(ScreenIntPoint{745, 1056},
+                         {0, 10, 17, 29, 29, 33, 33, 0, 31, 27, 13});
+  CHECK_VELOCITY(Up, 2.3, 2.7);
+}

gfx/layers/apz/test/gtest/TestScrollHandoff.cpp

@@ -567,6 +567,12 @@ TEST_F(APZScrollHandoffTester, ScrollgrabFlingAcceleration1) {
   SCOPED_GFX_PREF_FLOAT("apz.fling_min_velocity_threshold", 0.0f);
   SCOPED_GFX_VAR(UseWebRender, bool, false);
   CreateScrollgrabLayerTree(true /* make parent scrollable */);
+
+  // Note: Usually, fling acceleration does not work across handoff, because our
+  // fling acceleration code does not propagate the "fling cancel velocity"
+  // across handoff. However, this test sets apz.fling_min_velocity_threshold to
+  // zero, so the "fling cancel velocity" is allowed to be zero, and fling
+  // acceleration succeeds, almost by accident.
   TestFlingAcceleration();
 }