Statement#

On macOS, the frame parameter alpha fades the whole frame: text, mode line, header line and minibuffer. The background-only parameter alpha-background that Linux builds support does not work on the macOS (NS) port in Emacs 30, Emacs 31 or emacs-plus. So no part of a macOS GUI frame, such as a status bar, can stay solid while the rest is see-through. A terminal frame in a terminal with background transparency (such as Ghostty) differs: only cells with the default background are see-through, and cells with their own background color stay solid.

Evidence#

Checked 2026-09-26 while setting up a see-through Emacs GUI on macOS; the choice left was a solid frame (opacity 100), the whole frame see-through, or the terminal frame.