Outputs can be toggled without DPMS, but only on and off are provided for DPMS. Strangely, Sway does respond to output dpms toggle but it seems to just do the same thing as output dpms off.

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

Scale has a few options which are not documented, scale-title-filter isn't documented on the wiki either.

Waybar keeps crashing when clicking the idle inhibitor module. On sway it seems to work so that leads me to believe it is a problem with river not implementing the idle inhibit protocol.

I'm fine if you decide to not support the protocol, but could there be a little note about

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