That looks great! Indeed, the tiles should overflow to the sides. That implementation seems perfect.
As for configuration, my preference would be x and y percentages, rather than hard resolution values. I use another, high-resolution screen to set everything up before starting AM on the CRT TV and with percentages it would work on both displays with the same settings. Plus it's... simpler with percentages, I guess?
Thanks for such quick development and for the great stuff you make!