This is a video of Mikhail (Splash main maintainer) live-coding a "twitter popup closer".
This video shows a code editor for Splash scripts with syntax highlighting and autocompletion, and a live browser window with web developer tools enabled. Editor allows to execute browser automation scripts, changes are displayed live in the browser view.
Splash is written in 4 different programming languages (Python, Lua, JS, and you have to know C++ to debug some of the issues), and has 3 different event loops integrated (Twisted, QT and WebKit). It was not enough, so now we have 4th event loop - IPython's ZeroMQ event loop - integrated with all the others. Sounds like a good idea, right? ;)
There is a custom IPython kernel for Lua using Lua-Python bridge, with Splash integration and a custom Lua code completer. Common IPython stuff like inline image screenshots is supported. Code completer is relatively smart - it knows about "live" objects, about some Splash specifics, and it is syntax aware to some extent (though not much).