Open RetroArch frameless / chromeless

I want to write a launcher app like EmulationStation, ie. a standalone program that starts RetroArch with a game (currently via CLI using the -L flag). It’s a bit of a hack, but to overlay the emulator with my own UI (when the user pauses), I simply want to lay my semi-transparent app over RetroArch.

To do that I want to run RetroArch filling the whole screen, but not in fullscreen mode, because if RetroArch is in Fullscreen-Mode I can’t just put another app on top of that. But if I run it in windowed mode and simply resize the window to fill the whole screen it will show the frame of the window.

Is it possible to start RetroArch in window mode but do not show the frame of the window?

Thank you Lukas

settings > video > windowed fullscreen mode