Yeah, I mean, I get what you're saying about the API design. That's probably more important to people because it's fun or novel, but it basically does have retained state about individual widgets. Not literally every individual widget, but it needs to store something, or it wouldn't know what you hovered over, or were dragging and dropping, so it does this with IDs. That's state, and it's retained. That is, it's not flushed after every frame. There's a global state used to track this, but it's just hidden from you. The IDs are probably generated based on call order, since the design requires you to "check" by drawing.
Like you said though, even immediate mode rendering in reality worked a bit differently than people really understood. Today we're not actually performing a DMA transfer with every single call.
You're generally right about how compositors do effects layering, since you have to continually reference existing texture data to composite layers without limits, though as far as I know, web browsers do this a bit differently, and I'm not sure what the limits are with the major implementations, or if they exist.
Like you said though, even immediate mode rendering in reality worked a bit differently than people really understood. Today we're not actually performing a DMA transfer with every single call.
You're generally right about how compositors do effects layering, since you have to continually reference existing texture data to composite layers without limits, though as far as I know, web browsers do this a bit differently, and I'm not sure what the limits are with the major implementations, or if they exist.