Opt-in Liquid Glass look for the macOS desktop app

[builtbyd3v]

The iOS app already uses Liquid Glass and it looks really good. On desktop the Glass opacity slider only blends the app's own surfaces, and the window itself stays opaque, so there's nothing behind the glass to actually see.

I want to build an opt-in version for the macOS app with two background options:

• see-through: native macOS vibrancy, so whatever is behind the window (other apps, the desktop) shows through blurred
• wallpaper: pick an image and the glass sits on top of that instead
• reuse the existing Glass opacity slider for strength, no new slider
• off by default, and it follows Reduce Transparency and Increase Contrast
• macOS only for now, Windows and Linux stay as they are

I saw #8643 was closed because native blur isn't wanted as a new appearance feature, and that wallpapers are already being discussed in #6903, so I'm asking before I write any code. Would you take this if it's macOS only and off by default? If only one of the two modes is ok, I'd start with see-through.