>> If the menu-bar isn't displayed then we could display the Hamburger icon
>> in the tool-bar, and clicking on it will pop-up the menu with items from
>> the menu-bar, so users won't need to display both menu-bar and tool-bar.
>
> If the menu bar isn't displayed, C-mouse-3 already displays a menu
> that shows all the top-level menu-bar items.  So we don't really need
> the Hamburger icon (but it wouldn't hurt to have it, of course --
> provided they don't disable the tool bar as well...)

Yes, this is useful when they don't disable the tool bar,
then the Hamburger menu will look like this: