Before adding more animations into niri, I'm making a "visual tests" application. It shows a set of hardcoded scenarios which I can quickly go through and visually check that everything looks right.
It uses solid color rectangles as "windows", but otherwise this is the real niri layout code and real niri + Smithay rendering code, drawing to a GTK GL area.
For example, on the last test you can see that my offscreen code currently clips CSD shadows (during the open animation).
visual tests are such a lifesaver, so glad I stole that idea from osu!lazer