Multicursor disappears when not moving

还没有人认领这个 Issue。

## 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](https://matplotlib.org/stable/gallery/widgets/multicursor.html) may be used for demonstration:
```python

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

```

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。

由索引模型根据 Issue 内容生成。