Just a heads up if anyone else uses VirtualBox and manages to get Seamless Mode stuck on. I was able to find a solution.
- Shut down all your virtual machines, if they're running, and quit VirtualBox.
- Open the ~/Library/VirtualBox/Machines folder. Find the folder with the VM that's stuck in Seamless Mode. Open this one.
- In here, you'll see a few folders along with a configuration XML file. Open the XML file.
- Edit the line that contains Seamless - it'll say "True", change that to "False".
- Save and close the XML file.
- Restart VirtualBox.