This issue is about the Persistent List View experiment (#28637), currently only available in the Site Editor.

The List View can be toggled with a Ctrl+Alt+O shortcut (Shift+Alt+O on Win).
The list is navigable with the arrow keys, and selecting an item will move the focus to the corresponding block.

There is no way to move the focus back to the List View, though (unless tabbing all the way back to it).
The workaround is to toggle the sidebar off and on again, but it's obviously not ideal.