Walker,
The Share button, unfortunately, behaves a little different in every scenario. What browser are you using? In most, it should actually be a popup that goes above the window entirely. In some though, it uses an in-page pop-up, which can get cut-off by the window.
Also, if you can post (or Private Message) us the URL of the site, we can also take a look with the browser you're using and see if there's any changes that might help.
Thanks,
Alex