Floating bar in additional scrollable parent isn't placed correctly

[marijnh]

Since merging #16, if you have an editor in a scrollable parent with a scrollable top-level document, it is very easy to make the bar appear in an incorrect place. The way updateFloat now takes a scrollParent argument doesn't seem to work very well.

(Tested on firefox, by wrapping an editor in a 400px high overflow: auto div.)

cc @cjbest