Toolbar icons disappeared

首先，在所列出的 JupyterLab 2.2.2 和 ipympl 0.5.8 环境中，使用提供的 plt.figure() 和 plt.plot() 示例复现该问题。在检查所列出的 Jupyter 扩展配置的同时，将工具栏的渲染效果与附带的屏幕截图进行比较；当图标可见且现有按钮和工具提示仍然正常工作时，即表示完成。

Hello everyone,
I'm sorry to ask this as I thought this was already an issue a few weeks ago. But I looked through all the issues and couldn't find it. So maybe I was wrong or I was not searching properly. Sorry for the inconvenience.

Describe the issue

The toolbar icons have disappeared. This happened already a while ago after I reinstalled ipympl. So I thought it was just a bug in the version I updated to. But I updated ipympl and jupyter-matplotlib yesterday and still have no icons. Do you have any idea what I did wrong? The icons are the same as in the ipywidgets set, isn't it?

I have attached a screenshot at the bottom. All the buttons work just fine (also the tooltips), so it's just about the GUI.

Thanks for you help!

Versions

Output of

python -c "import sys; print('\n',sys.version); import ipympl; print('ipympl version:', ipympl.__version__)" ; jupyter --version ; jupyter nbextension list ; jupyter labextension list

 3.6.10 (default, Mar  5 2020, 10:17:47) [MSC v.1900 64 bit (AMD64)]
ipympl version: 0.5.8
jupyter core     : 4.6.3
jupyter-notebook : 6.1.4
qtconsole        : not installed
ipython          : 7.16.1
ipykernel        : 5.3.4
jupyter client   : 6.1.7
jupyter lab      : 2.2.2
nbconvert        : 6.0.7
ipywidgets       : 7.5.1
nbformat         : 5.0.7
traitlets        : 4.3.3
Known nbextensions:
  config dir: C:\Users\maxke\.jupyter\nbconfig
    notebook section
      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
      codefolding/main enabled
      - Validating: ok
      python-markdown/main enabled
      - Validating: ok
      hinterland/hinterland enabled
      - Validating: ok
      hide_input/main enabled
      - Validating: ok
      snippets/main enabled
      - Validating: ok
      code_prettify/2to3 disabled
    tree section
      nbextensions_configurator/tree_tab/main enabled
      - Validating: problems found:
        - require?  X nbextensions_configurator/tree_tab/main
    edit section
      codefolding/edit enabled
      - Validating: ok
  config dir: C:\Users\maxke\.conda\envs\jlab\etc\jupyter\nbconfig
    notebook section
      jupyter-matplotlib/extension enabled
      - Validating: ok
      nbdime/index disabled
      jupyter-js-widgets/extension enabled
      - Validating: ok
JupyterLab v2.2.2
Known labextensions:
   app dir: C:\Users\maxke\.conda\envs\jlab\share\jupyter\lab
        @jupyter-widgets/jupyterlab-manager v2.0.0 enabled  ok
        @jupyterlab/debugger v0.2.0 enabled  ok
        @jupyterlab/toc v4.0.0 enabled  ok
        @krassowski/jupyterlab_go_to_definition v1.0.0 enabled  ok
        @lckr/jupyterlab_variableinspector v0.5.1 enabled  ok
        jupyter-matplotlib v0.7.4 enabled  ok
        nbdime-jupyterlab v2.0.0 enabled  ok
        spreadsheet-editor v0.4.0 enabled  ok

Reproduce

plt.figure()
plt.plot(np.linspace(0,1, 100), np.random.rand(100))