Plugin Development: File System Access API is unavailable

in plugin ui iframe:

console.log(window.showSaveFilePicker, window);
// undefined Window {window: Window, self: Window, document: document, name: 'Inner Plugin Iframe', location: Location, …}

in figma desktop app console:

console.log(window.showSaveFilePicker, window);
// ƒ showSaveFilePicker() { [native code] } Window {0: Window, 1: Window, window: Window, self: Window, document: document, name: '', location: Location, …}