(Figure from my UIST 2012 poster)
A running program is interactive and alive. To determine whether it works, you just poke it and see what happens.
Unfortunately, to fix a problem, you must edit its source code, which is dead and unresponsive. Locating misbehavior in something static requires simulating a computer in your head.
I presented presented at the CHI conference in 2014. The paper and slides are here.
You can install Theseus in the Brackets IDE by opening its built-in extension manager and searching for “Theseus”. Theseus's source code is on GitHub. You should post bug reports and feature requests there.
Theseus is being developed as an extension for the Adobe Brackets code editor because it's just so gosh darn easy.