Although this thread is quite old, it may still be beneficial for those who are using greybox. Here is an update that could help:
Changing the GB_overlay position to "fixed" as suggested by Yoel in the gb_styles.css file solved the issue with greyboxes that are not full screen (such as gb_page_center). However, a full-screen greybox like GB_showPage was still causing horizontal scrollbars and expanding the browser window on Chrome, Edge, Chrome on iOS, and Safari on iOS.
Adjusting the GB_overlay width to 100% did not make a difference for non-full screen greyboxes, and did not resolve the issue for full-screen greyboxes either. Instead, setting the GB_overlay max-width to 100% successfully eliminated the problem of the browser window expanding and the appearance of horizontal scroll bars in Chrome, Edge, Chrome on iOS, and Safari on iOS.
Similar to Kevin's experience, Firefox did not encounter these problems with greybox, and these css modifications to the GB_overlay had no impact on Firefox, which always displayed the greybox correctly.