[Bug]: Intermittent `ValueError: The passed figure is not managed by pyplot` with ipympl (`%matplotlib widget`)

Nadie ha tomado este issue todavía.

### Bug summary

When using Matplotlib with ipympl (`%matplotlib widget`) in JupyterLab, I intermittently get:

```
ValueError: The passed figure is not managed by pyplot
```

This happens with standard pyplot usage (`plt.figure`, `plt.subplots(num=fig)`, `plt.sca(ax)`), without explicitly constructing canvases.

### Code for reproduction

```Python
%matplotlib widget
import time
import matplotlib.pyplot as plt

def try_once():
fig_num = 1
plt.close(fig_num)
fig = plt.figure(fig_num)

fig, axs = plt.subplots(10, 10, num=fig, squeeze=False)

for ax in axs.flat:
plt.sca(ax) # <-- intermittently raises
plt.plot([1, 2, 3])
fig.canvas.draw_idle()
time.sleep(0.001)

for i in range(300):
try:
try_once()
except Exception as e:
print('FAILED at iteration', i, '->', repr(e))
raise
```

### Actual outcome


```
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
Cell In[2], line 20
18 for i in range(300):
19 try:
---> 20 try_once()
21 except Exception as e:
22 print('FAILED at iteration', i, '->', repr(e))

Cell In[2], line 13, in try_once()
10 fig, axs = plt.subplots(10, 10, num=fig, squeeze=False)
12 for ax in axs.flat:
---> 13 plt.sca(ax) # <-- intermittently raises
14 plt.plot([1, 2, 3])
15 fig.canvas.draw_idle()

File ~/ipympl-debug/issue/.venv/lib/python3.12/site-packages/matplotlib/pyplot.py:1376, in sca(ax)
1372 # Mypy sees ax.figure as potentially None,
1373 # but if you are calling this, it won't be None
1374 # Additionally the slight difference between `Figure` and `FigureBase` mypy catches
1375 fig = ax.get_figure(root=False)
-> 1376 figure(fig) # type: ignore[arg-type]
1377 fig.sca(ax)

File ~/ipympl-debug/issue/.venv/lib/python3.12/site-packages/matplotlib/pyplot.py:995, in figure(num, figsize, dpi, facecolor, edgecolor, frameon, FigureClass, clear, **kwargs)
993 root_fig = num.get_figure(root=True)
994 if root_fig.canvas.manager is None:
--> 995 raise ValueError("The passed figure is not managed by pyplot")
996 elif (any(param is not None for param in [figsize, dpi, facecolor, edgecolor])
997 or not frameon or kwargs) and root_fig.canvas.manager.num in allnums:
998 _api.warn_external(
999 "Ignoring specified arguments in this call because figure "
1000 f"with num: {root_fig.canvas.manager.num} already exists")

ValueError: The passed figure is not managed by pyplot
```

### Expected outcome

Mosaicked plots will be shown without errors.

### Additional information

### Observed state at failure

At the moment the exception occurs:
- `root_fig.canvas.manager is None`
- but the corresponding `FigureManager` still exists in `matplotlib._pylab_helpers.Gcf` (so pyplot still manages the figure)

So pyplot’s “managed figure” check is making a false negative.

### Where the exception comes from

`plt.sca(ax)` calls `pyplot.figure(fig)` internally.
When passed a `Figure`, `pyplot.figure()` currently checks only:

```python
root_fig = num.get_figure(root=True)
if root_fig.canvas.manager is None:
raise ValueError("The passed figure is not managed by pyplot")
```

In this bug, `canvas.manager` is temporarily/incorrectly `None` even though the manager still exists in `Gcf`.

### Proposed fix

If `root_fig.canvas.manager is None`, fall back to a `Gcf` lookup to find the manager whose `m.canvas.figure is root_fig`, and reattach it.

```python
manager = root_fig.canvas.manager
if manager is None:
for m in _pylab_helpers.Gcf.get_all_fig_managers():
if getattr(getattr(m, "canvas", None), "figure", None) is root_fig:
manager = m
root_fig.canvas.manager = m
break
if manager is None:
raise ValueError(...)
```

This same logic eliminates the failures in my environment (0/60) when applied as a small monkey patch around `pyplot.figure()`.

This resembles matplotlib/matplotlib#19380 (same structural cause: `FigureCanvasBase.__init__` sets `self.manager=None` after replacing `figure.canvas`).

### Operating system

Ubuntu 24

### Matplotlib Version

3.10.8

### ipympl Version

0.10.0

### Matplotlib Backend

widget

### Python version

Python 3.12.3

### Jupyter version

jupyter lab --version: 4.5.4

### Installation

uv

Ejecuta primero la reproducción de JupyterLab proporcionada con el backend de widgets. Después inspecciona matplotlib.pyplot.figure(), al que se llega mediante plt.sca(ax), y los gestores de Gcf descritos en el informe. Se considera completado cuando el trazado repetido de mosaicos termina sin ValueError y pyplot reconoce el gestor de figuras existente.

Escrito por el modelo de indexación a partir del texto del issue.