See swaywm/sway#5299 (comment)

Would it be possible to have Peek automatically start the render after capture, saving it using the default filepath it already uses?

When wl-copy is sent a signal to terminate it, it should clean up the temp buffer files instead of leaving them around.

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.