While working on testcases for bug 1848957 I noticed that wheel actions are failing when the origin is pointer. Here a simplified Marionette testcase:

    def test_origin(self):
        wheel_chain = self.marionette.actions.sequence("wheel", "wheel_id")
        wheel_chain.scroll(15, 30, 20, 40, origin="pointer").perform()

The failure that results from that call to scroll():

ERROR testing/marionette/harness/marionette_harness/tests/unit/test_actions_wheel.py TestWheelAction.test_origin - marionette_driver.errors.InvalidArgumentException: Expected x to be finite number
stacktrace:
	RemoteError@chrome://remote/content/shared/RemoteError.sys.mjs:8:8
	WebDriverError@chrome://remote/content/shared/webdriver/Errors.sys.mjs:189:5
	InvalidArgumentError@chrome://remote/content/shared/webdriver/Errors.sys.mjs:380:5
	assert.that/<@chrome://remote/content/shared/webdriver/Assert.sys.mjs:485:13
	assert.number@chrome://remote/content/shared/webdriver/Assert.sys.mjs:204:43
	assertInViewPort@chrome://remote/content/shared/webdriver/Actions.sys.mjs:2142:15
	dispatch@chrome://remote/content/shared/webdriver/Actions.sys.mjs:1156:21
	dispatch/pendingEvents<@chrome://remote/content/shared/webdriver/Actions.sys.mjs:1836:14
	dispatch@chrome://remote/content/shared/webdriver/Actions.sys.mjs:1835:39
	dispatch/chainEvents<@chrome://remote/content/shared/webdriver/Actions.sys.mjs:1761:27
	dispatch@chrome://remote/content/shared/webdriver/Actions.sys.mjs:1763:7
	performActions@chrome://remote/content/marionette/actors/MarionetteCommandsChild.sys.mjs:501:23

Turns out that there is an issue in the WebDriver classic specification around the calculation of the target coordinates.

I've filed a new issue for it: https://github.com/w3c/webdriver/issues/1758

Unless it's clear how it should work this bug is blocked. Lets discuss it next week when James is back.

As discussed the current design in the WebDriver specification is not great. But it might also be hard to change it at this time given that it would cause backward incompatibilities. Also there could be multiple pointers chains so it would not be clear which position actually to take.

In the short term we probably should just fail with an unsupported operation if origin="pointer" has been specified for a scroll action. I can provide a patch for the classic specification, and also add a test for both classic and BiDi.

For a longer term solution I added an agenda item to the TPAC discussion.

The PR for WebDriver classic is up at https://github.com/w3c/webdriver/pull/1760.

Depends on D187391

https://hg.mozilla.org/mozilla-central/rev/984cc66d1fc1
https://hg.mozilla.org/mozilla-central/rev/d3a6da63a125