It looks pretty much stable on my machine but how do you diagnose figure out frame drops?

Frame rate obviously never going to be stable, also second while is kind of useless, it can just be an if. You are going to have occasional frame rate drops and even spikes in a real world scenario, thus enter the time_step to make things look smoother.

i see a solid 16.66xxxx something frame time in your demo in both fullscreen and windowed modes.