SearchKit - Improve checkbox UX for bulk actions

[colemanw]

Overview

Improves selection of checkboxes for SearchKit bulk actions.

Before

• No way to quickly select more than one row.
• No way to de-select all without first selecting every row.

After

• You can shift-click to quickly select multiple rows.
• The "select all" checkbox shows a minus sign when some but not all records are selected, and clicking it will deselect them.
• There is a dropdown next to the "select all" checkbox for finer control.
  

Technical Details

This incidentally fixes a bug where the selection would leak into other search displays if there was more than one on the page, per #22887.

[civibot]

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[eileenmcnaughton]

The styling is pretty funky under shoreditch (still testing)

[colemanw]

@eileenmcnaughton I updated the css for shoreditch. Now it looks like this:

[eileenmcnaughton]

I tested that I could select & export a page & that I could select & export all & it worked - it was out of order when using 'download spreadsheet' - which was odd but I didn't investigate as I think it doesn't relate