Strange refresh issue with Chromium on Gentoo

My system is Gentoo 64bit, with KDE4. I have installed chromium and have been using it for a long time without any problem. But recently, I noticed a strange issue. For example, in a text box, if I type hello, only hell will be displayed. The last o will be displayed only if I do something that could cause page update, like scroll up/down, hover my mouse over a link. This problem also happens with other input controls. I have tried to re-emerge the package, but it does not fix this problem. I really have no idea how to trouble this issue. There's no error log at all.