See swaywm/sway#5299 (comment)

This is a very simple shell protocol only allowing for fullscreen surfaces. You can find a very basic implementation here: https://github.com/swaywm/wlroots/blob/master/examples/fullscreen-shell.c

Many tiling window managers can be setup to change the active window whenever the cursor moves, to the window currently under the cursor. I'd find it useful to have an option like this in river too.
This is most useful on laptops, where swiping with thumb is often faster than a keyboard shortcut.