In Google Chrome and SRWare Iron (both built on Webkit): the window with grey border and "Edit" button advances too much horizontally in pages and posts. See images.
Img 1 - Theme: Thematic. Browser: SRWare Iron.
Img 2 - Theme: Twenty-ten. Browser: SRWare Iron.
In Opera and Firefox it works well.
I hope this information will help you to improve your wonderful plugin.