We seem to have a bit of a problem now.
It turns out that the reason F6 and F8 became 'broken', is that they are reserved by GTK for stock navigation when a splitter pane is involved in the UI.
It turns out that F6 is used to switch between current panes, and F8 to switch to the splitter itself to allow for keyboard resizing.
I don't think we can just arbitrarily remap those, since it's part of the accessibility project and was designed to coincide with the use of keys on other platforms/toolkits.
So I think we're now faced with the decision to either not allow any GtkPaned widgets in our UI, or to give up F6 and F8.
As a follow-up item, we probably need to do something I've been thinking of for a while and list out the different keys and how they're used on different platforms and in different applications. One thing tweaking things a bit for me is that on OS X, F9-F12 are used for basic OS-wide features, and other can be also (especially on laptops). http://guides.macrumors.com/Keyboard_shortcuts (the three links at the bottom of the page are quite helpful)