Nesting into HBox collapses plot width

Nobody has claimed this yet.

Here's another effect of requiring the canvas size explicitly, but somehow isn't visible unless the plot gets dropped into an ipywidgets layout (e.g. by a downstream package).

If the interactive matplotlib is the child of one p-Panel, it's fine. But gets squashed as the child of two p-Panels (regardless of orientation).

Explicitly setting canvas.layout.width fixes it, and propagates to the front-end as expected.

```
from ipywidgets import HTML, HBox, VBox
from matplotlib.figure import Figure
from ipympl.backend_nbagg import Canvas, FigureManager

figure = Figure()
canvas = Canvas(figure)
canvas.manager = FigureManager(canvas, 0)

axes = figure.add_subplot(1,1,1)
HBox([HBox([HTML(''), canvas])])
```

```
HBox([canvas])
```


Start with the provided Canvas/FigureManager reproducer in a notebook, comparing a canvas in one HBox with the same canvas nested in two HBoxes. Investigate how the canvas size is propagated to ipywidgets layouts; done means the plot keeps its width when nested without explicitly setting canvas.layout.width.

Written by the indexing model from the issue text.