An FFGL mixer that keys this layer's colour 0 over the layer below the way an Amiga genlock did, on the computer's own free-running pixel clock instead of the video's. The key lands a pixel away from its fill, so every edge carries a fringe, the fringe crawls as the clocks drift, and a failing sync rolls the overlay. None of it is drawn. It's just what a key timed off the wrong clock does.
This demo runs the plugin's shader in your browser, so it needs JavaScript and WebGL2. The plugin itself, its source and its downloads are on its repository.