Main thesis: it’s promise rejected with win.PopupNotifications is undefined on ExtensionsUI.jsm:385.
Side thesis: call from tab works well, or… I called it on focus event which is not listed at reference. Although, I got request popup, accepted it and my extension got permissions. Promise then called both methods - reject(arg) and resolve(arg) - in such order - first with <unavailable> arg, second with true. Is this how it should work?
UPD side thesis: reject() contains console.log(arg) and printed
<unavailable>
into browser console as I already said, but it also printed this into a page console:
permissions.request may only be called from a user input handler
It also has been said but now we know what exactly was given to reject().