This is a known issue. During beta testing with the RTL version of MSN Messenger (RTL: Languages that read from right to left such as Arabic and Hebrew), there were multiple problems that Patchou encountered and unfortunatly the feature had to be disabled for MSN Messenger versions that used RTL text.
This is a post from Patchou during our beta testing stage.
Originally posted by Patchou
I'm very sorry about that but what I said in the changelog was the truth: the RTL version of Messenger is programmed differently in some places and you've already seen for yourself what it was doing in tabbed chats. I won't give up on it but for now, I don't see any quick solution for it.
The top option was not working better, it was cutting the bottom of the window too, which is not acceptable.
Please refrain from making posts regarding Tabbed Chats being disabled in RTL versions of MSN Messenger as Patchou is working on trying to find a work around for it.