inconsistent scroll simulation

Description

When using the following code, each time the scroll goes roughly to the same place but not exactly, and there is no way to know exactly how much the element was scrolled:

private void scrollTo(int startX, int startY, int endX, int endY) {
    PointerInput finger = new PointerInput(PointerInput.Kind.TOUCH, "finger");
    Sequence scrollAction = new Sequence(finger, 0);
    scrollAction.addAction(finger.createPointerMove(Duration.ofMillis(0), PointerInput.Origin.viewport(), startX, startY));
    scrollAction.addAction(finger.createPointerDown(PointerInput.MouseButton.LEFT.asArg()));
    scrollAction.addAction(finger.createPointerMove(Duration.ofMillis(500), PointerInput.Origin.viewport(), endX, endY));
    // repeat last action as if to make the finger stay in place for 500 ms
    scrollAction.addAction(finger.createPointerMove(Duration.ofMillis(500), PointerInput.Origin.viewport(), endX, endY));
    scrollAction.addAction(finger.createPointerUp(PointerInput.MouseButton.LEFT.asArg()));
}

scrollTo(540, 1579, 540, 172);

On BrowserStack I see the correct request arrives:

[{"id":"finger","type":"pointer","parameters":{"pointerType":"touch"},"actions":[{"duration":0,"x":540,"y":1579,"type":"pointerMove","origin":"viewport"},{"button":0,"type":"pointerDown"},{"duration":500,"x":540,"y":172,"type":"pointerMove","origin":"viewport"},{"duration":500,"x":540,"y":172,"type":"pointerMove","origin":"viewport"},{"button":0,"type":"pointerUp"}]}]

but still each time I run that code on a fresh instance I get a bit different result.

Environment

• Appium version 1.21.0 (on BrowserStack)
• Android version 9.0, Google Pixel 2, UiAutomator2

Details

Please provide more details, if necessary.

Link To Appium Logs

https://gist.github.com/itaibh/d8028aa2f13ce3d16e15194101e9327e

In the Appium logs you'll see I use the Cancel action, but the result is the same as with lifting the finger.

Start by reproducing the scrollTo pointer-action sequence from the issue on Android 9 with UiAutomator2, comparing repeated runs and the linked Appium logs. Trace how the Java client sends the W3C pointer actions and determine whether the inconsistent scroll is introduced by the client or the remote driver. Done means the cause is identified and the behavior is made deterministic or the limitation is documented.