Huge console output turns UI unresponsive.
I guess just a random project with verbosity set to Diagnostic will render enough output to make the task runner explorer unresponsive and unable to cancel the execution.
I'll try a couple of tests with this, but I suspect the problem here his out of our control. When it comes to actually executing the tasks, we just tell Visual Studio what to do (provide a task definition of sorts), and the execution is carried out by VS itself. Once you actually begin to run a task, the extension is a bit out of the loop.
I'll run a couple of experiments and see if there's anything we can do from our end, but I suspect this is a factor of how Visual Studio implements the Task Runner Explorer.