Bug 1786452 - Add a gtest. r=hiro

Differential Revision: https://phabricator.services.mozilla.com/D156767

gfx/layers/apz/test/gtest/APZTestCommon.h

@@ -156,6 +156,10 @@ class MockContentController : public GeckoContentController {
   MOCK_METHOD1(CancelAutoscroll, void(const ScrollableLayerGuid&));
   MOCK_METHOD2(NotifyScaleGestureComplete,
                void(const ScrollableLayerGuid&, float aScale));
+  MOCK_METHOD4(UpdateOverscrollVelocity,
+               void(const ScrollableLayerGuid&, float, float, bool));
+  MOCK_METHOD4(UpdateOverscrollOffset,
+               void(const ScrollableLayerGuid&, float, float, bool));
 };
 
 class MockContentControllerDelayed : public MockContentController {

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

@@ -1599,6 +1599,42 @@ TEST_F(APZCOverscrollTester, SmallAmountOfOverscroll) {
 }
 #endif
 
+#ifdef MOZ_WIDGET_ANDROID  // Only applies to WidgetOverscrollEffect
+TEST_F(APZCOverscrollTester, StuckInOverscroll_Bug1786452) {
+  SCOPED_GFX_PREF_BOOL("apz.overscroll.enabled", true);
+
+  ScrollMetadata metadata;
+  FrameMetrics& metrics = metadata.GetMetrics();
+  metrics.SetCompositionBounds(ParentLayerRect(0, 0, 100, 100));
+  metrics.SetScrollableRect(CSSRect(0, 0, 100, 1000));
+
+  // Over the course of the test, expect one or more calls to
+  // UpdateOverscrollOffset(), followed by a call to UpdateOverscrollVelocity().
+  // The latter ensures the widget has a chance to end its overscroll effect.
+  InSequence s;
+  EXPECT_CALL(*mcc, UpdateOverscrollOffset(_, _, _, _)).Times(AtLeast(1));
+  EXPECT_CALL(*mcc, UpdateOverscrollVelocity(_, _, _, _)).Times(1);
+
+  // Pan into overscroll, keeping the finger down
+  ScreenIntPoint startPoint(10, 500);
+  ScreenIntPoint endPoint(10, 10);
+  Pan(apzc, startPoint, endPoint, PanOptions::KeepFingerDown);
+  EXPECT_TRUE(apzc->IsOverscrolled());
+
+  // Linger a while to cause the velocity to drop to very low or zero
+  mcc->AdvanceByMillis(100);
+  TouchMove(apzc, endPoint, mcc->Time());
+  EXPECT_LT(apzc->GetVelocityVector().Length(),
+            StaticPrefs::apz_fling_min_velocity_threshold());
+  EXPECT_TRUE(apzc->IsOverscrolled());
+
+  // Lift the finger
+  mcc->AdvanceByMillis(20);
+  TouchUp(apzc, endPoint, mcc->Time());
+  EXPECT_FALSE(apzc->IsOverscrolled());
+}
+#endif
+
 class APZCOverscrollTesterMock : public APZCTreeManagerTester {
  public:
   APZCOverscrollTesterMock() { CreateMockHitTester(); }