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...but yeah. As an end-user, all I can say is "you KNOW when something is written in Kirigami because it's got that nagging ugliness to it that you lack the UI design skill to diagnose, but you can sense".
Generally, I think they all fall under "It looks like a mobile app that's pretending to be a desktop app (the "Window settings for firefox" header, the part that darkened, and the modal) has been embedded in a frame inside a native QWidget app (the window decorations and the action buttons at the bottom) and you can tell.
...maybe it's a mix of the inconsistent font size and the way the modal isn't centred on the darkened region and evokes memories of X11 top-level windows when the window manager has died.