There have been several occasions where I wished we had a GUI debugging tool for the executor that allows to single-step through work method calls and that allows us to control multiple ESPs, so we can reproduce distributed situations like "late cancel". It turns out this is relatively easy to do, so I'm planning to submit a PR for this feature soon.
The GUI will have one window for every fragment instance (the master fragment instance and then one for each ESP fragment instance). This window will allow to step one subtask (work method invocation, see class ExSubtask) at a time, or it will allow to set breakpoints at specific subtasks, or one can resume the instance without the GUI. I'm also adding a very primitive queue display, but that will need more work, in the first version it won't be possible to look at the data in the executor queues.