Ah, when we start a smooth scroll animation the `mLastSampleTime` is initialized to `TimeStamp::Now()` [here](https://searchfox.org/mozilla-central/rev/b59a99943de4dd314bae4e44ab43ce7687ccbbec/gfx/layers/apz/src/AsyncPanZoomController.cpp#3485) instead of using the test sample time in the CompositorBridgeParent. So in some cases the first animation sampling's time delta is not what the test expects it to be.
Bug 1062609 Comment 8 Edit History
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Ah, when we start a smooth scroll animation the `mLastSampleTime` is initialized to `TimeStamp::Now()` [here](https://searchfox.org/mozilla-central/rev/b59a99943de4dd314bae4e44ab43ce7687ccbbec/gfx/layers/apz/src/AsyncPanZoomController.cpp#3485) instead of using the test sample time in the CompositorBridgeParent. So in some cases the time delta of the animation's first sampling is shorter than what the test expects it to be.