Setting dropdown width to 'initial' expands to the full width

[jasongrout]

This expands the dropdown to the full width possible, rather than just sizing to content:

from ipywidgets import *
Dropdown(layout={'width': 'initial'}, options=['very very very very very very very very very very very very very very long option', 'another option'])

It looks like the inner select box has a flex grow attribute:

ipywidgets/packages/controls/css/widgets-base.css

Line 663 in e1af986

flex: 1 1 var(--jp-widgets-inline-width-short);

, so it grows to be the full width. Taking off the inner select flex grow makes the content in the above example instead just expand to the width of the content. This is still a slight problem since the dropdown arrow overwrites the last bit of content - perhaps we should add a bit of padding or something to ensure the svg dropdown arrow is to the right of the last bit of content.

Since this changes the layout behavior, I consider this a breaking change for a user, so setting this to be a major release issue.

This issue was found when answering #2050

[jasongrout]

This is still a slight problem since the dropdown arrow overwrites the last bit of content - perhaps we should add a bit of padding or something to ensure the svg dropdown arrow is to the right of the last bit of content.

This is now: #2052

[jasongrout]

Setting to max-content works great, as in #2050. Thanks!