An electrochromic window that becomes transparent or opaque depending on whether it is currently turned on.

"Vitrine"
Plaza View is a room. "Your uncle's apartment, on loan to you for viewing the parade and celebrations today. This would be more of a sacrifice on his part if he weren't currently yachting around Corfu."
The smart window is a device in Plaza View. It is fixed in place. "A vast smart window [if transparent]overlooks the park[otherwise]has turned to a sheet of hazy blue[end if]." The smart window can be transparent. The smart window is transparent. The description is "An electrochromic device which changes shade and transparency in response to the application of current.
Curtains are so last year."

Note the "can be transparent" line. Devices ordinarily are not allowed to have transparency or opaqueness, but we can make an exception in this case. Without that line, attempts to change the transparency of the window will fail.

Carry out switching off the window: now the window is transparent.
Carry out switching on the window: now the window is opaque.
Instead of searching a transparent window: say "Isn't it lovely out there?"
Instead of searching an opaque window: say "The window is currently darkened."
Test me with "look through window / switch window / look through window / look".
Test me with "look through window / switch window / look through window / look".
Plaza View
Your uncle's apartment, on loan to you for viewing the parade and celebrations today. This would be more of a sacrifice on his part if he weren't currently yachting around Corfu.

A vast smart window overlooks the park.

>(Testing.)

>[1] look through window
Isn't it lovely out there?

>[2] switch window
You switch the smart window on.

>[3] look through window
The window is currently darkened.

>[4] look
Plaza View
Your uncle's apartment, on loan to you for viewing the parade and celebrations today. This would be more of a sacrifice on his part if he weren't currently yachting around Corfu.

A vast smart window has turned to a sheet of hazy blue.