[UX] Show bulk operations only when items have been selected, and hide filters when that happens

This was inspired by https://www.drupal.org/project/drupal/issues/3070558 ...our Drupal brethren have this issue in D9 with the new Claro admin theme:

The bulk operation form takes space on top of the form and in some cases pushes the content of the view below the fold, meaning that to see any content on the page, the user has to scroll

How they intent to solve this issue is to:

Move the bulk operation form below the view content. After a user has clicked a checkbox, make the bulk operations form position fixed. After user has clicked a checkbox the bulk operations form becomes the primary actions on the page and there's a strong reason to believe that the user wants to trigger an action. This will also add some clarity on the connection to checkboxes, as well as make it easier to activate an action.

Here's what the currently proposed solution looks like:

I like the idea of making the bulk operations sticky, so that the user can still see them as they scroll through and select additional items, but I thought that a better solution would be to have the bulk operations be hidden while there is nothing selected (which will save vertical space) ...once the user has ticked at least one checkbox, it is clear that the intention is to perform bulk operations, so the filters in that case become irrelevant for the task at hand. So my proposal is to make it so that only one set of controls is shown at each given time: either filters (no checkboxes ticked) or bulk operations (one or more checkboxes ticked).