Edit: Flexiglass is now updated to support Mavericks.
If you use the Flexiglass App (spoiler alert: you should) and upgraded to OSX Mavericks yesterday, you were greeted with:
On OSX Mountain Lion, clicking "Open Universal Access pane" took you to:
But OSX Mavericks doesn't have the option to "Enable access for assistive devices":
So I searched:
And found that the Apps are now given access on an app-by-app basis:
So simply check the Flexiglass App and you're back to sweet, sweet, window-snappity goodness.