Action for moving a pointer device by an offset

Hi,

I'm busy implementing a local end for the Actions part of the spec. In
Selenium, there's a method for moving the primary pointer input source by a
given offset:

https://github.com/SeleniumHQ/selenium/blob/selenium-3.0.1/java/client/src/org/openqa/selenium/interactions/Actions.java#L286

The interesting thing to note is that this simply updates the current input
state for the pointer input source --- it doesn't need an target webelement
to be set.

In the spec
<https://www.w3.org/TR/webdriver/#dfn-process-a-pointer-move-action>, it's
considered an error if the element is not specified, which means that the
existing selenium API can't be implemented.

I think this suggests we should amend the spec to allow this behaviour, but
that means that we'll need to set a starting input state for all input
devices, and call out what happens if the viewport were to scroll (from a
user's point of view, the pointing device remains stationary (it's screen
position doesn't change), even if the position within the DOM (and the
currently active element) does change).

Thoughts?

Simon