Ah, I see. This is due to the transparent background mode. Unlike Chrome browser, this plugin had the background set to transparent, which allows you to overlay the rendered data over GM layers (something you can't do with some other browser plugins).

However, in some cases this causes ghosting (exactly the same issue when you don't set GM background layers).  I think this can be fixed by clearing the buffer between frames if you don't want transparent rendering.  I will explore this later.

 Have released a new version, 1.3, which defaults to non-transparent, so you won't see ghosting. But in order to retain the ability to do transparent rendering, I've included a chromium_setup_bgtransparent() function that you would run before creating a chromium instance.