Comment 8 for bug 683084

Yes, it updates dynamically.. kind of.

I have my WM set to resize opaquely, not using an outline or something, so when I resize my window then the application actually receives configure events and repaints the window. In this situation, if I raise or lower the top edge of the window then the local menubar will appear or disappear.

If you're doing something that the WM has control of, this isn't quite the case. If you grab the titlebar of a window and start dragging it around, the client application never receives a configure event until you release the mouse. So if you drag your window above or below the threshold you won't see immediate visual feedback until you release your mouse.