Nesting into HBox collapses plot width

还没有人认领这个 Issue。

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])
```


在 notebook 中从提供的 Canvas/FigureManager 复现器开始，将一个位于单个 HBox 中的 canvas 与同一个嵌套在两个 HBoxes 中的 canvas 进行比较。调查 canvas 大小如何传播到 ipywidgets 布局；完成标准是：在未显式设置 canvas.layout.width 的情况下，图表在嵌套后仍保持其宽度。

由索引模型根据 Issue 内容生成。