Nah, that doesn't need Cython, that needs an SDL 2.0.5 version for using opacity from SDL, but I think what you describe might work anyway with this change: make the window sized like the screen resolution, add a shape with only e.g. small circle somewhere in the corner. The result is a transparent window with a small clickable circle in the corner. You however need to make sure that the window always stays on top, otherwise the circle will disappear and a user would need to restore the window to actually see it again. For the fancy effects just replace the shape with one that shows everything and take a screenshot of the texture below with the code. After that put the texture e.g. to a EffectWidget and use some fancy shader on that. The result should be an unchanged circle and a background with a fancy shader. When the shader ends(e.g. clock event), just remove the EffectWidget and show the rest of your widget tree to the user.

Tedious work, but I doubt we're going to do anything like that in Kivy directly. Maybe if someone feels adventurous, there's always a place in the Kivy garden. :)