The app is a virtual keyboard: https://github.com/archisman-panigrahi/vboard
So it must be unfocused to be useful. On GNOME it somehow sets up automatically. On KDE it adds a window rule. For Niri I can have "open-focused false", but it only affects the initial state.
Always wondered why Plasma set my Laptop screen to a logic DPI of 150, instead of the "standard" 96. So here's the code:
It detects if the screen is internal or external and it's size, and uses different target DPIs for laptop, phone, external monitor and TV. So a button would have different physical size in those screens, but assuming your viewing distance is also different, would result in a similar "perceived" size.
And in my case, the code somehow fails to detect a laptop lid device, so it uses phone target DPI.