CSS class 'jupyter-widgets' should not appear node 'jp-OutputArea-output'

[qzchenwl]

package	version
jupyterlab	1.0.2
jupyterlab-manager	1.0.1

When customizing my own widget, I need to calculate the output area width. If the node 'jp-OutputArea-output' has css attribute 'overflow: visible', I cannot get the correct width.

And there's no means to change css attribute of node 'jp-OutputArea-output', because it's my parent node.

Without 'jupyter-widgets' class, the node 'jp-OutputArea-output' has css attribute 'overflow: auto' which is fine.

[jasongrout]

Closed by #2500.