Got a proper highlight shader working in last year's jam game after taking the time to do it. Handmade in both OpenGL3.0 and DirectX12.
It draw the object to highlight in a separate frame buffer, then detects its edges, then blurs those edges.
I wanted to do it during the jam, but somehow I didn't manage to make it before the end. At the time, it just displayed the object as unlit when hovered.
Swimming Brick was originally a short underwater adventure game, and with the later refinements, its map got expanded with mechanisms and intricate tunnels!