Verified on Windows whenever the debugger is active, meaning that a program is running.  (Debug On just means that it will become active when code is run.) No stepping is needed; debugger can be pointing to the inital docstring line.  For me also, Idle stops and has to be externally closed, as opposed to totally disappearing by itself.

The doc for Stack Viewer says "Show the stack traceback of the last exception".   Example:

>>> 1/0
Traceback (most recent call last):
  File "<pyshell#0>", line 1, in <module>
    1/0
ZeroDivisionError: division by zero
>>> 

Selecting Stack Viewer pops up a viewer box.  This still works after 
[DEBUG ON]
>>>
turns the debugger on but inactive.  Entering anything at the prompt disables viewing the 'last' exception, contrary to my understanding of the short doc.  So I might add '(if no other code has been run)' to the doc.

Selecting Stack Viewer while a program is running (sleeping in this next example)

>>> import time; time.sleep(10); 1/0

brings up a box after the exception is printed.  So 'last exception' can actually be 'next exception'.  But in this case, the user process is left 'running' and no '>>> ' prompt appears, and one must Shell -> Restart to do anything further.  This is not good behavior.

When one selects Debug -> Debugger while user code is running, Idle brings up a message box "Don't debug now: You can only toggle the debugger when idle".  I think Debug -> Stack Viewer should be similarly disabled, though perhaps graying out the menu entry might be better. It could also be grayed out when there the 'last exception' cannot be viewed because other code has been run.  Stack Viewer should definitely be ignored when the debugger is active, and I see no need to let people select it *before* an exception occurs and the prompt is displayed.  The next menu entry, Auto-open Stack Viewer, takes care of opening upon future exceptions.