Fixing size of inputs via box-shadow

[cchaos]

The box-shadow that mimics the input’s border is now inset. This means the 40px height is actually 40px and the box-shadow ensure it's borders don't spill over the 400px width.

It's very subtle, hence the zoomed in gif. I also tweaked the second box-shadow slightly to have a better edge to it.

This also touched the EuiComboBoxList styles since it was doing extra css to account for the extended border.

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This originally was noticed as an issue when I saw this:
https://github.com/elastic/kibana/blame/master/src/core_plugins/input_control_vis/public/components/editor/controls_tab.less

So cc'ing @nreese : This will fix the problem you were running into with inputs inside of accordions. When this get's merged into Kibana, can you remove this extra style? It will cause visual issues with the accordion if for some reason it doesn't correctly get the height of 0. See example:

[nreese]

created elastic/kibana#19927 in the kibana repo to track the issue. Thanks for looping me in.

[snide]

Awesome