When you press the Zoom window button you get an absolutely unexpected size and position of the window. Nothing to do with the window content. Moreover, if you press again the Zoom window button the window remains as it was left, in a strange place and size, not returning to its previous geometry.
Window zoom is an application-level feature, not an engine-level feature. See <http://webkit.org/blog/101/back-to-basics/> for more about the distinction. Bugs in Safari should be reported at http://bugreport.apple.com/. Closing as INVALID per our process for non-WebKit bugs in Bugzilla.