Similar experience in the past but seem fixed now. I guess it should be a gvim problem. You may test by comparing with other gtk based text editors such as geany or gedit.
It should be `lines' not set correctly. try :set lines? for gvim, lines depends on both gfn and window height. -- regards, ==================================================== GPG key 1024D/4434BAB3 2008-08-24 gpg --keyserver subkeys.pgp.net --recv-keys 4434BAB3 唐詩226 王維 送別 山中相送罷 日暮掩柴扉 春草明年綠 王孫歸不歸