I haven't looked into the specifics, but it seems like Buster reuses one single frame for all browser tests.
Some of my tests do things to the objects like document, where reseting everything to the initial state can be difficult. Creating a new iframe per test should be pretty cheap.
It's not that cheap. It would really slow down.
This could be implemented as some kind of plugin, but not as the default.
Like @fabiomcosta says, that's going to significantly hurt performance. It's also a non-trivial change. We've discussed having a debug mode for this at some point, but it'll be post-1.0.
Not sure if you took that into account when evaluating the idea, but you should be able to create empty iframes and modify them from within the existing test document/frame, rather than sending a new request to the buster server for each iframe. The tests could be added as non-evaluated code in the "main frame" and then document.writed or something into the iframes.
I still don't know how this would fit into Buster of course, but if I get some time over to check it out myself, where should I start? buster-core? buster-server?
Hah, I didn't consider that. I think to do that, you'd have to hook into the test runner. Unfortunately, it's not directly pluggable like that, but I guess you could somehow overwrite runTest