`breakpoint()` not working

Empieza reproduciendo el problema en el entorno mínimo de conda descrito, comparando breakpoint() con import pdb; pdb.set_trace() en una celda de notebook. Después, sigue cómo ipykernel gestiona estos puntos de entrada del depurador. Se considera terminado cuando breakpoint() invoca pdb en el entorno de JupyterLab indicado y el comportamiento existente queda cubierto frente a regresiones.

Using a fresh install of Jupyter Lab my usual breakpoint() to invoke the debugger in a Notebook is skipped (see screenshot). Using import pdb; pdb.set_trace() instead invokes pdb as expected.

I was expecting (am I wrong?) that a call to breakpoint() in a cell would invoke pdb.

This is using a fresh conda install of Jupyter Lab this morning, no reconfiguration of the environment has occurred.

Description

Reproduce

I've created a fresh minimal environment:

$ conda create -n tmp_env python=3.8 jupyterlab

$ jupyter --version
jupyter core     : 4.7.1
jupyter-notebook : 6.4.3
qtconsole        : not installed
ipython          : 7.27.0
ipykernel        : 6.3.1
jupyter client   : 7.0.2
jupyter lab      : 3.1.10
nbconvert        : 6.1.0
ipywidgets       : not installed
nbformat         : 5.1.3
traitlets        : 5.1.0

and then I run jupyter lab and try to call breakpoint() as noted in the screenshot above.

I used jupyter lab --debug and scanned the output, nothing looked amiss (but there's a lot and I'm not sure what to look for) - I can attach a copy if needed?

I noticed this originally in a second more complex environment (also built today, making me think this is a library update issue somewhere) and I made the above simple environment to try to narrow things down, but the issue remains.

Environment

Linux Cinnamon using Anaconda with Firefox, the OS is all up to date.

$ inxi -b
System:    Host: ian-XPS-15-9550 Kernel: 5.4.0-77-generic x86_64 bits: 64 Desktop: Cinnamon 4.8.6 
           Distro: Linux Mint 20.1 Ulyssa 
Machine:   Type: Laptop System: Dell product: XPS 15 9550 v: N/A serial: <superuser/root required> 
           Mobo: Dell model: 0N7TVV v: A00 serial: <superuser/root required> UEFI [Legacy]: Dell v: 1.9.0 date: 10/11/2018

originally posted by @ianozsvald on https://github.com/jupyterlab/jupyterlab/issues/10996