I'd say stay, but make sure that if they cut you off quickly you either have some $$$ saved up to sustain yourself or another job prospect to go to.

Anyway, don't worry about your job stagnating. It's just programming... career progression is usually lackluster anyway (don't know your case personally) Also you can always get creative with your CV later on (as you know, work expands to fill the allocated slot).

Think about this you're getting paid to work on your code! What could be better? Just cover your tracks and don't get into any IP dispute over your code ;-)

Edit: If you wanted to use the time for something that could possibly look good on your CV or your employer would appreciate, you can look for some internal software problem to solve, Or make a demo app using your employer's technology stack.