Bug 1480866 - Avoid creating a fling animation with zero velocity. r=kats

Differential Revision: https://phabricator.services.mozilla.com/D2743

--HG--
extra : moz-landing-system : lando

gfx/layers/apz/src/AndroidFlingPhysics.cpp

@@ -151,6 +151,8 @@ void AndroidFlingPhysics::Init(const ParentLayerPoint& aStartingVelocity,
                                float aPLPPI)
 {
   mVelocity = aStartingVelocity.Length();
+  // We should not have created a fling animation if there is no velocity.
+  MOZ_ASSERT(mVelocity != 0.0f);
   const double tuningCoeff = ComputeDeceleration(aPLPPI);
   mTargetDuration = ComputeFlingDuration(mVelocity, tuningCoeff);
   MOZ_ASSERT(!mTargetDuration.IsZero());

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -3064,6 +3064,15 @@ ParentLayerPoint AsyncPanZoomController::AttemptFling(const FlingHandoffState& a
     residualVelocity.y = 0;
   }
 
+  // If we're not scrollable in at least one of the directions in which we
+  // were handed velocity, don't start a fling animation.
+  if (GetVelocityVector().Length() < gfxPrefs::APZFlingMinVelocityThreshold()) {
+    // Relieve overscroll now if needed, since we will not transition to a fling
+    // animation and then an overscroll animation, and relieve it then.
+    aHandoffState.mChain->SnapBackOverscrolledApzc(this);
+    return residualVelocity;
+  }
+
   // If there's a scroll snap point near the predicted fling destination,
   // scroll there using a smooth scroll animation. Otherwise, start a
   // fling animation.