Partial or Glitchy Interactivity

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## Partial or Glitchy Interactivity
In my dev. environment, interactive figures:
- fail to show resize handle, figure header and footer are never displayed
- do not respect (but do not error) settings for:
- fig.canvas.header_visible
- fig.canvas.footer_visible
- fig.canvas.resizable
- completely disappear on mouse click or drag (no toolbar option selected) and never return on selecting toolbar Reset option
- toolbar panning/zooming work fine
- playing with figure.set_frameon(False) seems to cure the disappearing figure issue

My production environment (with [as far as I can tell] same versions) works as expected:
- shows and respects resize handle, figure header and footer are displayed
- respects settings for:
- fig.canvas.header_visible
- fig.canvas.footer_visible
- fig.canvas.resizable
- does not disappear on mouse click or drag (no toolbar option selected)
- toolbar panning/zooming work fine

Both environments are using the classic notebook experience (not Lab), the biggest/most obvious difference being that production is a TLJH installation (i.e. JupyterHub spawning single-user notebooks), but all the notebook package versions across environments appear to be in sync (see below), so unless spawning affects things silently, I'd be surprised if that's the issue.

I've borrowed and executed this code (from https://github.com/matplotlib/ipympl/issues/203) as a test to make sure there's nothing wonky in my code and because it incorporates the ioff/ion pattern I'm reliant on:

```
%matplotlib widget
from IPython.display import display # this is automatically imported in the notebook anyway
from ipywidgets.widgets import HBox
import matplotlib.pyplot as plt
from scipy.misc import face

plt.ioff() # turn off interactive mode so figure doesn't show
fig1, ax1 = plt.subplots()
fig2, ax2 = plt.subplots()
plt.ion() # figure still doesn't show

ax1.imshow(face())
ax2.imshow(face(True))

display(HBox([fig1.canvas, fig2.canvas]))
```
Executing this code exhibits all of the issues I've described, i.e. it does not work (as described above) in development, but does work in production.

## Versions
### Development environment:
```
3.6.9 (default, Jan 15 2020, 16:16:15)
[GCC 6.3.0 20170516]
ipympl version: 0.5.6
jupyter core : 4.6.3
jupyter-notebook : 6.0.3
qtconsole : not installed
ipython : 7.14.0
ipykernel : 5.3.0
jupyter client : 6.1.3
jupyter lab : not installed
nbconvert : 5.6.1
ipywidgets : 7.5.1
nbformat : 5.0.6
traitlets : 4.3.3
Known nbextensions:
config dir: /home/dschofield/.jupyter/nbconfig
notebook section
jupyter-js-widgets/extension enabled
- Validating: OK
config dir: /home/dschofield/.local/share/virtualenvs/goldmine-hCKJOvFR/etc/jupyter/nbconfig
notebook section
jupyter-matplotlib/extension enabled
- Validating: OK
voila/extension enabled
- Validating: OK
jupyter-js-widgets/extension enabled
- Validating: OK
qgrid/extension enabled
- Validating: OK
JupyterLab v1.0.0
Known labextensions:
app dir: /home/dschofield/.local/share/jupyter/lab
@jupyter-widgets/jupyterlab-manager v1.1.0 enabled OK
bqplot v0.5.1 enabled OK
ipyevents v1.7.0 enabled OK
ipysheet v0.4.3 enabled OK
ipytree v0.1.3 enabled OK
ipyvolume v0.5.2 enabled OK
jupyter-leaflet v0.11.4 enabled OK
jupyter-matplotlib v0.4.2 enabled OK
jupyter-threejs v2.1.1 enabled OK
jupyter-vue v1.0.0 enabled OK
jupyter-vuetify v1.1.1 enabled OK
jupyterlab-datawidgets v6.2.0 enabled OK
pywwt v0.7.0 enabled OK
```

### Production environment:
```
3.6.7 | packaged by conda-forge | (default, Feb 28 2019, 09:07:38)
[GCC 7.3.0]
ipympl version: 0.5.6
jupyter core : 4.6.3
jupyter-notebook : 6.0.3
qtconsole : not installed
ipython : 7.14.0
ipykernel : 5.3.0
jupyter client : 6.1.3
jupyter lab : 1.2.5
nbconvert : 5.6.1
ipywidgets : 7.5.1
nbformat : 5.0.6
traitlets : 4.3.3
Known nbextensions:
config dir: /opt/tljh/user/etc/jupyter/nbconfig
notebook section
jupyter-matplotlib/extension enabled
- Validating: OK
nbresuse/main enabled
- Validating: OK
voila/extension enabled
- Validating: OK
jupyter-js-widgets/extension enabled
- Validating: OK
qgrid/extension enabled
- Validating: OK
tree section
jupyter_server_proxy/tree enabled
- Validating: OK
JupyterLab v1.2.5
Known labextensions:
app dir: /opt/tljh/user/share/jupyter/lab
@jupyter-widgets/jupyterlab-manager v1.1.0 enabled OK
jupyter-matplotlib v0.7.2 enabled OK
```
I'm assuming that the differences in Lab extensions are not of concern here because I'm not using Lab, but before I go about testing that assumption, expert insight is welcomed.
Thanks in advance.

Inizia eseguendo la riproduzione fornita di %matplotlib widget con le versioni di sviluppo e produzione elencate, quindi confronta le estensioni Notebook e le estensioni Lab mostrate nel report. Il lavoro è completato quando le figure interattive mantengono il loro header, footer, resize handle e le impostazioni di visibilità, e non scompaiono più dopo un'interazione con il mouse o Reset.

Scritto dal modello di indicizzazione a partire dal testo della issue.