Nesting into HBox collapses plot width

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.