(Before anyone asks -- I've installed the Guest Additions correctly. :] ) When I turn on on Seamless Mode for VirtualBox, it seems to work, but my windows no longer have any borders, and so I can't move them, maximize them, etc.
Why is this? Is there a fix?
Seemless mode will not work with compositing on. Whatever method they use to detect the windows can't recognize the borders. Turn off Visual Effects and you'll have borders again.
Here is the bug report if you want to track it: http://www.virtualbox.org/ticket/6167