MPL Event Handling Doc Example not fully functioning

まだ誰も着手していません。

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.


JupyterLab 3.0.7 と ipympl 0.7.0 を使って、MPL のイベント処理ドキュメントにあるドラッグ可能な矩形の演習を再現し、その後 QT5Agg と比較します。ドラッグ中の draw_artist と blit の呼び出しを追跡します。Jupyter 環境でバーが表示されたままドラッグ可能であれば完了です。

索引モデルが issue の本文から書いたものです。