Am I correct that if I add a RenderEffect on a

graphicsLayer

of a canvas that this only applies to the current canvas, and not what is behind? So, I can't make a panel that blurs everything that is behind it? If true, is there a way in Compose currently to achieve this?

That is correct, since it wouldn't have access to those pixels

So would be impossible to create a panel that blurs anything that is behind it, except its own canvas?

The way the rendering pipeline works, correct. You can create a custom modifier to set on the composable that needs to be blurred, optionally with mask area.