keepsSourceVisible property
iOS 26+: whether the button is left out of the menu's morph.
By default the system morphs the button into its menu: the button disappears as the menu grows out of it and morphs back in when the menu closes. With this on, the menu grows out of a clear stand-in at the button's place instead, so the button is never hidden by the morph and is simply there again the moment the menu closes. The menu still opens at the button's place, so while it is up it covers the part of the button under it. macOS menus never hide their button.
Implementation
final bool keepsSourceVisible;