Clearing cell while in (i)pdb causes lockout

This is a very simple and reproducible bug.

Consider a cell that had `ipdb.set_trace()`. It works as expected and gives you the ability to interact with `ipdb`. However, if you do not *explicitly* exit the debugger (say, you clear the output or rerun the cell), the whole notebook freezes.

System: Mac OS with version 5.4.1

Reproduce the issue in a notebook cell containing `ipdb.set_trace()` on the reported Jupyter Notebook version, then clear the output or rerun the cell without explicitly exiting the debugger. Trace the notebook's debugger and kernel handling to identify the lockout; done means clearing or rerunning no longer freezes the notebook.

Written by the indexing model from the issue text.