Multicursor disappears when not moving

首先在 JupyterLab 和 classic Notebook 中运行链接的 Matplotlib MultiCursor 文档示例，然后将其行为与 QT 进行比较。调查光标小部件的渲染路径；当鼠标停止移动后，多光标在两个 Jupyter 环境中都仍保持可见时，即表示完成。

Describe the issue

In both JupyterLab and old Notebook MultiCursor not only is blinking when is bing moved, but also completely disappears when mouse stops motion. Same code works fine in QT.

Documentation example may be used for demonstration:

import numpy as np
import matplotlib.pyplot as plt
from matplotlib.widgets import MultiCursor

t = np.arange(0.0, 2.0, 0.01)
s1 = np.sin(2*np.pi*t)
s2 = np.sin(4*np.pi*t)

fig, (ax1, ax2) = plt.subplots(2, sharex=True)
ax1.plot(t, s1)
ax2.plot(t, s2)

multi = MultiCursor(fig.canvas, (ax1, ax2), color='r', lw=1)
plt.show()

Versions

3.8.3 | packaged by conda-forge | (default, Jun  1 2020, 17:43:00) 
[GCC 7.5.0]
ipympl version: 0.6.3
jupyter core     : 4.7.1
jupyter-notebook : 6.2.0
qtconsole        : 5.0.2
ipython          : 7.20.0
ipykernel        : 5.3.4
jupyter client   : 6.1.7
jupyter lab      : 3.0.7
nbconvert        : 6.0.7
ipywidgets       : 7.6.3
nbformat         : 5.1.2
traitlets        : 5.0.5
Config option `kernel_spec_manager_class` not recognized by `ListNBExtensionsApp`.
Known nbextensions:
  config dir: /home/ilyap/.jupyter/nbconfig
    notebook section
      limit_output/main disabled
      table_beautifier/main disabled
      jupyter-js-widgets/extension  enabled 
      - Validating: OK
      qtconsole/qtconsole disabled
      hinterland/hinterland disabled
      snippets_menu/main disabled
      jupyter-datawidgets/extension  enabled 
      - Validating: OK
      collapsible_headings/main disabled
      code_font_size/code_font_size disabled
      addbefore/main disabled
  config dir: /home/ilyap/anaconda3/etc/jupyter/nbconfig
    notebook section
      bqplot/extension  enabled 
      - Validating: OK
      ipyvolume/extension  enabled 
      - Validating: OK
      jupyter-webrtc/extension  enabled 
      - Validating: OK
      jupyter-datawidgets/extension  enabled 
      - Validating: OK
      jupyter-matplotlib/extension  enabled 
      - Validating: OK
      jupyter-threejs/extension  enabled 
      - Validating: OK
      k3d/extension  enabled 
      - Validating: OK
      jupyter-js-widgets/extension  enabled 
      - Validating: OK
      qgrid/extension  enabled 
      - Validating: OK
      hide_code/hide_code  enabled 
      - Validating: OK
Config option `kernel_spec_manager_class` not recognized by `ListLabExtensionsApp`.
[W 2021-03-01 14:01:32.364 LabApp] Config option `kernel_spec_manager_class` not recognized by `LabApp`.
JupyterLab v3.0.7
/home/ilyap/anaconda3/share/jupyter/labextensions
        jupyter-matplotlib v0.8.3 enabled OK
        bqplot v0.5.21 enabled OK
        jupyterlab-datawidgets v7.0.0 enabled OK
        @jupyter-widgets/jupyterlab-manager v3.0.0 enabled OK (python, jupyterlab_widgets)

Other labextensions (built into JupyterLab)
   app dir: /home/ilyap/anaconda3/share/jupyter/lab
        @arbennett/base16-one-dark v0.1.5 enabled OK
        jupyterlab_hidecode v0.1.4 enabled OK


Uninstalled core extensions:
    @aquirdturtle/collapsible_headings