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.
I presented presented at the CHI conference in 2014. The paper and slides are here.
(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.
Theseus's ongoing mission is to turn code into an interface for exploring the run-time behavior of a program. It does this by
Theseus is being developed as an extension for the Adobe Brackets code editor because it's just so gosh darn easy.