Comment 1 for bug 659155

I encountered this issue while upgrading 10.04.1 to 10.10 desktop as well. The installation halted with a prompt for "Configuring grub-pc" asking whether to proceed with or without installing grub. However, this window opened behind the Distribution Upgrade window, and at this point in the upgrade the windows had lost their title bars and borders, making them unable to be moved. Clicking the edge of the prompt window would not bring focus, nor would Alt-Tab. The installation was unable to proceed due to not being able to make selections in the "Configuring grub-pc" window.

The solution was to revert the machine to a previous state and re-attempt the upgrade, this time preemptively moving the Distribution Upgrade window to the edge of the display.