The clientX and clientY values are, as per the spec, the offset from
the viewport.
This makes them actually be that and also fixes up the calculations
for offsetX, offsetY, pageX and pageY.
I assume all of these got messed up in some sort of refactor in the
past.
The spec comment from the now-removed
compute_mouse_event_client_offset() function sadly has no convenient
place to be anymore so, for now, it is just gone as well.
Personally, I think it'd make sense to refactor a lot of this file so
that not every mouse event repeats a large chunk of (almost) identical
code. That way there'd be a nice place to put the comment without
repeating it all over the file.
But that is out of the scope of this PR.
Also: I know, offsetX and Y are not fully fixed yet, they still
don't ignore the element's CSS transforms but I am working on that
in a new PR.
2cbb691438