Fix ColumnsSelector search input should stay fixed above menu list

[WiXSL]

Problem

Items in ColumnsSelector component can be placed above the search input.
This disrupts expected UX.

Solution

Modify markup to fix the position of the search input

Additional Checks

• The PR targets master for a bugfix or a documentation fix, or next for a feature
• The PR includes unit tests (if not possible, describe why)
• The PR includes one or several stories (if not possible, describe why)
• The documentation is up to date

[fzaninotto]

Strange: in stories (e.g., http://localhost:9011/?path=/story/ra-ui-materialui-list-columnsbutton--label-types), the search box appears sticked to the top of the popover.

This doesn't seem to happen in any of the demos, where there is proper top padding in the popover.

Any idea why?

[WiXSL]

Any idea why?

I believe its because Storybook renders the component with a very minimal theme and without the app-level overrides used by the demo.
The top spacing used to come from the MenuList. Once the search input was moved outside of the MenuList, that padding no longer applied to it.
In the demo, there’s still extra padding coming from theme/component overrides or global styles around the popover.

I've added some padding.

Before/After this PR (Story):

Before/After this PR (Demo):