How to smoothly fade out images and avoid flicker?

Hi, I'm only starting out on Jitter/GL so there must be something very naive I'm doing or something obvious I'm missing.
I want to display an image then smoothly fade it out, blending into the next image. My first try was using the framebuffer fade out via @erase_color's alpha, for simplicity. I'm enabling a jit.gl.layer with the image for a short hold time. Large hold times produce a smooth image fade, values around 2x frame period may flicker,
going shorter makes rendering definitely flicker, and values below half frame period mainly show black.
Before going further I want to understand why this happens. See this patch:

I sense that the simple framebuffer fadeout is too rigid, so I'm also happy about suggestions how to do this properly with several layers that are explicitly faded (which would allow fading in, too), or with shaders (whoooo!).

Many thanks!
Diemo