Browsers already have multiple layers of sandboxes. You can always add another one to the browser if you are paranoid.
Personally I prefer not running untrusted javascript in the first place (where I can avoid it). An exploit that gains control of the browser is already a doomsday scenario if you are using the browser for internet banking, work or anything that matters. Being able to grab the input and output of other applications is a nothingburger in comparison.
> Or under the threat model of company policy or legislation not allowing you to handle confidential information on a system that can't offer even a basic guarantee of privacy.
Those are usually fine with running countless proprietary drivers and tools with root access. So in other words it's security threater.
I also prefer running trusted code, but that's not an option most of the time (for varying definitions of trust)
99% of the apps I use have no business with the contents of other apps I run so why allow it? Because X11 can't do anything about it is not a valid reason.
Last I'd checked, Windows had the same security issues as X, so it's unlikely to come up.
Next to that Windows has an even stronger permission model regarding DRMed content. But thats only to please movie studios it seems...
Under the threat model of websites using browser exploits to grab other data on your system?
Or under the threat model of company policy or legislation not allowing you to handle confidential information on a system that can't offer even a basic guarantee of privacy.