Some UX recommendations:

• "Select Source" sounds like technical wording to me. Have you considered something like "Select Monitor..." or "Select Screen..."?
• Any button that toggles off a feature like the mouse or microphone should make their symbolic icon (or the whole button) red (in light mode) or orange (in dark mode). LibAdwaita's ultra-flat button styling always makes it nearly impossible for me to figure out what state something is in, activated or deactivated, and the simple \ bar in the icon isn't really noticeable enough for me.

I'm not sure what sort of interaction you're accepting from users, but for me the biggest frustration I have about Kooha is its handling of area-based recording. It's just so tricky to get a specific area. (And it doesn't look like I can select a full window, which would also be useful.)

The way Spectacle handles this is pretty much perfect. I'm not sure exactly what they do, but they might actually take a full-screen screenshot first, and display that as a borderless window, so it looks like you're looking at just your normal display, but you're actually seeing a frozen-in-time view of how it was when the screenshot was triggered. And then you can just select a rectangle right on the screen. Very nice.

Any progress on this? The non-resizable window looks really stupid even in english on stock KDE:
image

I know GNOME apps are not too much customizable in general, but at least give the user an option to force the resize...