Plot output with ipywidgets formatting incorrectly

Personne n'a encore pris cette issue.

## Describe the issue
I built a small GUI within a notebook and had two figures in an ipywidgets HBox object. the latest version of ipympl made it so I could not have both figures in an HBox and generally disrupted the formatting of the object. I was able to fix the issue by reverting both matplotlib and the ipympl versions.

a short script that generates the broken output is as follows
```python
%matplotlib widget
from IPython.display import display, HTML
display(HTML(".container { width:100%!important;}"))

import ipywidgets as ipy
import matplotlib.pyplot as plt
import numpy as np
out1 = ipy.Output()
b1 = ipy.Button(description = 'button1')
b2 = ipy.Button(description = 'button2')
x = np.linspace(0,7,21)
y = np.sin(x)
with out1:
fig,ax = plt.subplots()
ax.plot(x,y)

display(ipy.HBox([out1,ipy.VBox([b1,b2])]))
```

## Versions
(working version)
```
ipympl == 0.5.8
matplotlib == 3.3.3
```
(broken version)
```ipympl == 0.8.8
matplotlib == 3.5.1
```

Exécutez le script du notebook fourni avec les versions de matplotlib/ipympl fonctionnelle et défaillante indiquées, puis comparez la disposition de HBox et le formatage des widgets. La tâche est terminée lorsque la figure et les boutons s’affichent correctement ensemble dans HBox avec les versions concernées.

Rédigé par le modèle d'indexation à partir du texte de l'issue.