When you want to look at a particular slide within a case, click on the slide's thumbnail is it opens up next to the collection/case/slide navigation panel.

Depending on how PMA.slidebox is configured behind the scenes, additionally selected slides are opened next to the existing viewport, or replace the current viewport altogether.

If the above is true , then slides can be removed from the grid presentation by clicking them again, effectively de-selecting them.

The following screenshot shows a case with 5 out of 6 slides selected:

When a new slide is selected from a new case, the viewport is always cleared, and the slide takes up as much viewing area on the screen as possible.