ipywidgets 8 Stacked layout attribute is broken.

[ibdafna]

Using ipywidgets 8.0.0rc1.

The Stacked layout widget added in #2376 appears to be broken. I came across this issue when preparing the 8.0 changes notebook for the 2022 Scipy widgets tutorial.

from ipywidgets import Stacked
button = Button(description='Click here')
slider = IntSlider()
stacked = Stacked([button, slider])
stacked  # will show only the button"`

cc @jasongrout

[jasongrout]

Fixed in #3491, in d3b71f8