Transport change in MS Proposal

Martin,

In the MS proposal [1], a JS application has the ability to change the transport for a RealtimeMediaStream object, when detecting the transport bandwidth is too low through JS callback.
Since JS execution cannot be interrupted by callbacks, what happens if the callback is delayed by a long running JS block?

Thanks,
Li

[1] http://html5labs.com/cu-rtc-web/cu-rtc-web.htm#connectivity-checking