Re-executing a cell hides the figure when it is given a `num=...` argument

Describe the issue

Explicit figure numbers seem to cause a small glitch in ipympl. When the following cell is executed once, it shows the plot. When re-executing the cell, it disappears again.

x = np.linspace(-np.pi/2, np.pi/2)
plt.figure(num=1)
plt.plot(x, np.sin(x))

I've attached a working example: repeat_plot_issue.zip

The inline and notebook backends do not have this issue, which leads me to believe this is a bug. There are two workarounds:

1. Close the figure in the beginning of the cell with e.g. plt.close(1)
2. Show the figure at the end of the cell with plt.show()

(I'm using Jupyter notebooks with a group of about 40 students, and they got confused by this difference in behavior between the backends. I understand this is not a major issue because there are simple workarounds. Still, for newcomers, it is an extra difficulty: "Help, my plot has disappeared!")

Versions

 3.9.7 | packaged by conda-forge | (default, Sep 29 2021, 19:20:46) 
[GCC 9.4.0]
ipympl version: 0.8.2
Selected Jupyter core packages...
IPython          : 7.30.1
ipykernel        : 6.6.0
ipywidgets       : 7.6.5
jupyter_client   : 6.1.12
jupyter_core     : 4.9.1
jupyter_server   : 1.13.1
jupyterlab       : 3.2.5
nbclient         : 0.5.9
nbconvert        : 6.3.0
nbformat         : 5.1.3
notebook         : 6.4.6
qtconsole        : not installed
traitlets        : 5.1.1
Known nbextensions:
  config dir: /home/toon/.jupyter/nbconfig
    notebook section
      splitcell/splitcell disabled
      nbextensions_configurator/config_menu/main  enabled 
      - Validating: problems found:
        - require?  X nbextensions_configurator/config_menu/main
      contrib_nbextensions_help_item/main  enabled 
      - Validating: OK
      spellchecker/main  enabled 
      - Validating: OK
      toc2/main  enabled 
      - Validating: OK
      k3d/extension  enabled 
      - Validating: OK
    tree section
      nbextensions_configurator/tree_tab/main  enabled 
      - Validating: problems found:
        - require?  X nbextensions_configurator/tree_tab/main
  config dir: /home/toon/.local/etc/jupyter/nbconfig
    notebook section
      nbdime/index  enabled 
      - Validating: problems found:
        - require?  X nbdime/index
      rise/main  enabled 
      - Validating: OK
  config dir: /home/toon/miniconda3/envs/py4sci/etc/jupyter/nbconfig
    notebook section
      jupyter-matplotlib/extension  enabled 
      - Validating: OK
      rise/main  enabled 
      - Validating: OK
      voila/extension  enabled 
      - Validating: OK
      jupyter-js-widgets/extension  enabled 
      - Validating: OK
      nbextensions_configurator/config_menu/main  enabled 
      - Validating: problems found:
        - require?  X nbextensions_configurator/config_menu/main
      contrib_nbextensions_help_item/main  enabled 
      - Validating: OK
    tree section
      nbextensions_configurator/tree_tab/main  enabled 
      - Validating: problems found:
        - require?  X nbextensions_configurator/tree_tab/main
  config dir: /etc/jupyter/nbconfig
    notebook section
      jupyter-js-widgets/extension  enabled 
      - Validating: OK
JupyterLab v3.2.5
/home/toon/miniconda3/envs/py4sci/share/jupyter/labextensions
        k3d v2.11.0 enabled OK (python, k3d)
        jupyter-matplotlib v0.10.2 enabled OK
        @jupyter-widgets/jupyterlab-manager v3.0.1 enabled OK (python, jupyterlab_widgets)
        @ijmbarr/jupyterlab_spellchecker v0.7.2 enabled OK (python, jupyterlab-spellchecker)
        @ryantam626/jupyterlab_code_formatter v1.4.10 enabled OK (python, jupyterlab-code-formatter)
        @voila-dashboards/jupyterlab-preview v2.1.0 enabled OK (python, voila)

Bắt đầu bằng cách chạy ví dụ repeat_plot_issue.zip được đính kèm với backend ipympl và so sánh với các backend inline và notebook. Việc thực thi lặp lại ô chứa plt.figure(num=1) phải giữ cho hình vẫn hiển thị mà không yêu cầu plt.close(1) hoặc plt.show().