I've had the same issue. The icon doesn't just move when the window is resized, it also moves when the user scrolls (the position is tied to the window border rather than the page).

A more dynamic woffset would be nice to have since it would allow us to keep buttons next to page elements instead of having them shift around.

I gave up and kept the buttons at the top of the window because not all of my users have the same size monitors and that alone was enough to through off the positioning.