I don't really know how to describe this problem in English.
A picture can say more words - look at the tabs at the top of the linked image. The current font settings are shown as well.
I don't know what basic info I should provide so if something missing, please tell me.
GNOME Shell 3.4.1
I know, a reboot can make everything normal but I'm looking for a more permanent solution that can fix this problem (or bug).
Thanks for Fitoschido, I finally know what keyword I could use for googling. I think it's a bug of gnome shell.