MPL Event Handling Doc Example not fully functioning

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I tried the example from the MPL event handling docs: a [bar chart with draggable bars](https://matplotlib.org/stable/users/event_handling.html#draggable-rectangle-exercise) ("extra credit" version). This works fine in `QT5Agg` backend, but doesn't fully work in Jupyter Lab (v3.0.7) with ipympl (v0.7.0). The plot displays fine, and I can see that click and drag events are being received by the widget, but during drag, the bars vanish. In fact, if in the click handler I include a color change:

```python
self.rect.set(animated=True,color='red')
```

Then I can see a faint red outline after the bar is "dropped" in the correct location (see below). So clearly the click/drag/release events are working, but the bar drawing during and after drag via `draw_artist` and `blit` are not.


Riproduci l’esercizio del rettangolo trascinabile dalla documentazione sulla gestione degli eventi di MPL con JupyterLab 3.0.7 e ipympl 0.7.0, quindi confrontalo con QT5Agg. Traccia le chiamate a draw_artist e blit durante il trascinamento; il lavoro è completato quando le barre rimangono visibili e trascinabili nell’ambiente Jupyter.

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