The functionality for custom slide show is very far from usable, at least on Mac, and I would like to suggest the following improvements:

When adding more slides to a custom show, they are always placed at the end. It should be possible to define where to put the new slides, simply by selecting one of the slides already present - the added slides then should be placed directly after this slide in succession.

It should be possible to add multiple slides in one go.

When removing slides, the list should stay in place, instead of bumping to the top.

When repositioning slides, the list of slides scrolls to the top. The list should stay in place, and scroll only to keep the selected slide inside the viewport.