Comment 13 for bug 388508

Yes, it's definitely the HIG that is the problem. I would point out that none of those apps follow the HIG anyway. According to the guidelines, if there are more than 6 tabs, you should display a list control instead. http://library.gnome.org/devel/hig-book/stable/controls-notebooks.html.en

The real problem lies in the fact that that that guideline does not apply in this type of situation. That guideline is for pages of options, not for a tabbed document interface (as an implementation of multiple document interface). There is no interface guideline for multiple document interface, much less mdi via tabs.