Figures in Output disappear if Output displayed twice in same cell

还没有人认领这个 Issue。

## Describe the issue
Got an odd one.
If an Output contains a figure, and one calls a single cell containing `display(Output)` twice, the output "empties" and no figure is shown.

Example below. One can emphasise it by first calling `display(o)` in one cell, and then doing the double call (I press ctrl + Enter twice) in another cell. I attach a figure showing the result.

```python
# in one cell:
%matplotlib widget
import matplotlib.pyplot as plt
from ipywidgets import Output
o = Output()
with o:
plt.figure()
plt.plot([1,2,3])
# In a separate cell, call this twice after each other
display(o)
```
## Versions

```
3.8.2 | packaged by conda-forge | (default, Mar 23 2020, 17:32:17) [MSC v.1916 64 bit (AMD64)]
ipympl version: 0.5.6
jupyter core : 4.6.3
jupyter-notebook : 6.0.3
qtconsole : 4.7.2
ipython : 7.13.0
ipykernel : 5.2.0
jupyter client : 6.1.3
jupyter lab : 2.1.4
nbconvert : 5.6.1
ipywidgets : 7.5.1
nbformat : 5.0.4
traitlets : 4.3.3
Known nbextensions:
config dir: C:\Users\thomasaar\.jupyter\nbconfig
notebook section
default_cells disabled
config dir: C:\Users\thomasaar\.conda\envs\py5\etc\jupyter\nbconfig
notebook section
ipyvolume/extension enabled
- Validating: ok
jupyter-webrtc/extension enabled
- Validating: ok
jupyter-matplotlib/extension enabled
- Validating: ok
plotlywidget/extension enabled
- Validating: ok
jupyter-js-widgets/extension enabled
- Validating: ok
tree section
ipyparallel/main enabled
- Validating: ok
JupyterLab v2.1.4
Known labextensions:
app dir: C:\Users\thomasaar\.conda\envs\py5\share\jupyter\lab
@jupyter-widgets/jupyterlab-manager v2.0.0 enabled ok
@jupyter-widgets/jupyterlab-sidecar v0.5.0 enabled ok
jupyter-matplotlib v0.7.2 enabled ok```

使用提供的 Python notebook 示例重现该问题：在 Output widget 内创建 matplotlib 图形后调用 display(o) 两次。使用所列的 JupyterLab、ipympl、matplotlib 和 ipywidgets 版本比较第一次和第二次显示；当重复显示后图形仍然可见时，即表示完成。

由索引模型根据 Issue 内容生成。