Letting any GUI application capture all input and take full control of the desktop completely defeats the point of sandboxing and X11 does exactly that.
Yawn, X11 (and similar "unsecure" desktop environments) existed for half a century and the sky hasn't fallen. I'm tired of that "will somebody think of the children/grandparents" scare mongering.
> Defeats the point of sandboxing
Sandboxing defeats the point of said applications. If you want your computer to have no functionality, check out Figma. A clickable prototype sounds like precisely the security the world needs right now.