I got this same behavior on Windows 8 using Chrome 22.0.1229.94 and IE 10.
I'm seeing the issue using Firefox 16.0.2 on OSX Lion.
Very strange - I'm on same version here.
I'm going to add a width counter to give more feedback on the resizing, as seems to be a bit of a bottleneck.
It seems the minimum reported by window.innerWidth on 16.0.2 (OS X, 10.8.2) is 453. Once it hits that, I can drag it all the smaller I like, and it is stuck at 453.