Fix scroll animation UpdateTarget for zero-duration segments

BUG=645317
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_precise_blink_rel

Review-Url: https://codereview.chromium.org/2332923002
Cr-Commit-Position: refs/heads/master@{#418343}

cc/animation/scroll_offset_animation_curve.cc

@@ -207,18 +207,6 @@ void ScrollOffsetAnimationCurve::UpdateTarget(
     return;
   }
 
-  // The total_animation_duration_ is zero because of the delay that we
-  // accounted for when the animation was created. The new duration should
-  // also take the delay into account.
-  if (total_animation_duration_.is_zero()) {
-    DCHECK_LE(t, 0);
-    total_animation_duration_ =
-        SegmentDuration(new_target.DeltaFrom(initial_value_),
-                        duration_behavior_, base::TimeDelta::FromSecondsD(-t));
-    target_value_ = new_target;
-    return;
-  }
-
   base::TimeDelta delayed_by = base::TimeDelta::FromSecondsD(
       std::max(0.0, last_retarget_.InSecondsF() - t));
   t = std::max(t, last_retarget_.InSecondsF());
@@ -228,6 +216,15 @@ void ScrollOffsetAnimationCurve::UpdateTarget(
   gfx::Vector2dF old_delta = target_value_.DeltaFrom(initial_value_);
   gfx::Vector2dF new_delta = new_target.DeltaFrom(current_position);
 
+  // The last segement was of zero duration.
+  if ((total_animation_duration_ - last_retarget_).is_zero()) {
+    DCHECK_EQ(t, last_retarget_.InSecondsF());
+    total_animation_duration_ =
+        SegmentDuration(new_delta, duration_behavior_, delayed_by);
+    target_value_ = new_target;
+    return;
+  }
+
   double old_duration =
       (total_animation_duration_ - last_retarget_).InSecondsF();
   double old_normalized_velocity = timing_function_->Velocity(

cc/animation/scroll_offset_animation_curve_unittest.cc

@@ -237,7 +237,7 @@ TEST(ScrollOffsetAnimationCurveTest, CurveWithLargeDelay) {
   curve->UpdateTarget(-0.01, gfx::ScrollOffset(0.f, 300.f));
   double duration =
       ScrollOffsetAnimationCurve::SegmentDuration(
-          gfx::Vector2dF(0.f, 300.f),
+          gfx::Vector2dF(0.f, 200.f),
           ScrollOffsetAnimationCurve::DurationBehavior::INVERSE_DELTA,
           base::TimeDelta::FromSecondsD(0.01))
           .InSecondsF();
@@ -257,5 +257,55 @@ TEST(ScrollOffsetAnimationCurveTest, CurveWithLargeDelay) {
                       curve->GetValue(base::TimeDelta::FromSecondsD(1.0)));
 }
 
+// This test verifies that if the last segment duration is zero, ::UpdateTarget
+// simply updates the total animation duration see crbug.com/645317.
+TEST(ScrollOffsetAnimationCurveTest, UpdateTargetZeroLastSegmentDuration) {
+  std::unique_ptr<ScrollOffsetAnimationCurve> curve(
+      ScrollOffsetAnimationCurve::Create(
+          gfx::ScrollOffset(0.f, 100.f),
+          CubicBezierTimingFunction::CreatePreset(
+              CubicBezierTimingFunction::EaseType::EASE_IN_OUT),
+          ScrollOffsetAnimationCurve::DurationBehavior::INVERSE_DELTA));
+  double duration_in_seconds = kInverseDeltaMaxDuration / kDurationDivisor;
+  double delay_in_seconds = 0.02;
+  double curve_duration = duration_in_seconds - delay_in_seconds;
+
+  curve->SetInitialValue(gfx::ScrollOffset(),
+                         base::TimeDelta::FromSecondsD(delay_in_seconds));
+  EXPECT_NEAR(curve_duration, curve->Duration().InSecondsF(), 0.0002f);
+
+  // Re-target 1, this should set last_retarget_ to 0.05.
+  gfx::ScrollOffset new_delta =
+      gfx::ScrollOffset(0.f, 200.f) -
+      curve->GetValue(base::TimeDelta::FromSecondsD(0.05));
+  double expected_duration =
+      ScrollOffsetAnimationCurve::SegmentDuration(
+          gfx::Vector2dF(new_delta.x(), new_delta.y()),
+          ScrollOffsetAnimationCurve::DurationBehavior::INVERSE_DELTA,
+          base::TimeDelta())
+          .InSecondsF() +
+      0.05;
+  curve->UpdateTarget(0.05, gfx::ScrollOffset(0.f, 200.f));
+  EXPECT_NEAR(expected_duration, curve->Duration().InSecondsF(), 0.0002f);
+
+  // Re-target 2, this should set total_animation_duration to t, which is
+  // last_retarget_. This is what would cause the DCHECK failure in
+  // crbug.com/645317.
+  curve->UpdateTarget(-0.145, gfx::ScrollOffset(0.f, 300.f));
+  EXPECT_NEAR(0.05, curve->Duration().InSecondsF(), 0.0002f);
+
+  // Re-target 3, this should set total_animation_duration based on new_delta.
+  new_delta = gfx::ScrollOffset(0.f, 500.f) -
+              curve->GetValue(base::TimeDelta::FromSecondsD(0.05));
+  expected_duration =
+      ScrollOffsetAnimationCurve::SegmentDuration(
+          gfx::Vector2dF(new_delta.x(), new_delta.y()),
+          ScrollOffsetAnimationCurve::DurationBehavior::INVERSE_DELTA,
+          base::TimeDelta::FromSecondsD(0.15))
+          .InSecondsF();
+  curve->UpdateTarget(-0.1, gfx::ScrollOffset(0.f, 500.f));
+  EXPECT_NEAR(expected_duration, curve->Duration().InSecondsF(), 0.0002f);
+}
+
 }  // namespace
 }  // namespace cc