Function behind_window_blending 

pub fn behind_window_blending() -> bool

Whether the compositor blends a material behind a translucent window — AppKit’s .behindWindow blending, and Mica on Windows.

Read at runtime rather than from a cfg, because the Windows answer is a build number: DWMWA_SYSTEMBACKDROP_TYPE lands in build 22621, and below it gpui’s backend applies no backdrop while its renderer still clears the window transparent — the desktop then shows through unblurred. The build is read once.