The Popup menu is not accessible for customization.
For example, in other browsers, the Popup menu is a way to interact with the HTML element under the mouse.
And plugins can add new menu items to interact in new ways with the HTML page.

For example, my current need is:
I need to add an additional item in the popupmenu of a WebView.
I need this additional item to appear only when the clicked HTML element is (let's say) a picture.
The label of this additional menu item will depend on some details of the clicked HTML element.
And I need this HTML element to also be available in the callback of that menu item.

I would summarize the requirement as:
 - the Popup menu should have an API to programatically add/remove menu items depending on a click context
(the HTML element under the mouse, which button is clicked, keyboard modifiers involved, the webView involved...)
 - the callback of popup menu items should also have access to the click context (especially
the HTML element under the mouse, that the user probably wants to interact with).