[al_borland]

There is switchable glass that can change between transparent and opaque. It’s used for some car sunroofs and various other applications. While it’s not “at the same time”, as a theme idea for the OS that has analogs in the physical world, it could be done.

[brewdad]

I've seen this used for the restroom doors at trendy bars. The glass is clear until you lock the door. It then turns opaque while you do your business.

Apparently, solid doors made of steel or wood are too last century.

[msephton]

Hotel showers too. When they are transparent they make the room feel bigger.

[xattt]

The next UI direction for iOS should be coloured or textured glass (with bubbles or glass block refraction) with dark mode options that mimic uranium- or cerium-doped glass.

[what]

I would pay for windows that can do so that so I don’t need curtains.

[al_borland]

It looks like there are adhesive films to add the feature to existing windows, if you don’t want to go all-in on entire windows (which is also an option).