popup appears inside the menu’s panel and is given a fixed width.
…is written on https://developer.mozilla.org/en-US/Add-ons/WebExtensions/user_interface/Popups#Popup_resizing.
However, how large is that fixed width actually?
I need that information to show the user an error if they configure a too large size in my add-on. Also, of course, determinating whether that fixed-width popup is actually used would also help, but that is not obligatory (as I can, of course just show a warning that that may potentially too large).