It can only apply shader(s) to the current frame I think. To produce the crt ghosting you'd probably need access to the previous frame (not an expert).

I've tried the shaders in the following repo with ghostty. They definitely work. I ended up keeping a cursor trail shader. https://github.com/0xhckr/ghostty-shaders

Yes, correct! If you check out https://ghostty.org/docs/config/reference, the iPreviousCursor is available, so it can be used against the iCurrentCursor to produce a fading effect. But I think the entire previous framebuffer isn't there (yet).