The top-left is used: https://github.com/WebKit/WebKit/blob/main/LayoutTests/imported/w3c/web-platform-tests/resources/testdriver-vendor.js#L36 The spec says that the center should be used: https://www.w3.org/TR/webdriver2/#processing-actions
<rdar://problem/116145770>
As it turns out, currently we don't implement a sound interpretation of the `origin` value for the `viewport`/`pointer` cases. We do interpret `origin` when it is an Element, which is the third case in https://www.w3.org/TR/webdriver2/#processing-actions, but we do so incorrectly -- i.e. what the original bug report is about. So really there's two feature bugs and one bug fix in here. I'll make sure to reflect that distinction in a not-so-distant patch.