orientation="vertical" no longer works (ipywidgets 7).

[korsbo]

Sliders and selection sliders retain their horizontal orientation despite specifying vertical.

This problem occurred after I updated ipywidgets from version 6.0.0 to 7.1.0. I performed this upgrade in order to solve a "widgets not appearing" problem (#182), so a simple downgrade of ipywidgets would not help much.

I suspect that the underlying issue is the same as in both #198 and #197.

[korsbo]

I'll make a quick attempt to figure out what is causing these layout problems. I can however not make any promises of success, and any pointers would be appreciated.