`chrome.contextMenus` now have an entry point `chrome.contextMenus.onClicked` so we can know when the item is choosen, but there is no entry point to know when the menu is (trying to be) opened. `chrome.contextMenu.onOpening` or `chrome.contextMenu.onOpened` is required for an addon which provids a context menu feature based on very heavy preliminary operations. For example, my addon Text Link https://addons.mozilla.org/firefox/addon/text-link/ provides some menu items like "Open (URLs extracted from selection) in new tabs", "Copy (URLs) to the clipboard", and so on. However, the extraction operation is sometimes very heavy, so I don't want to prepare the list of URLs from selection when the selection is just changed - the user will open the context menu, but possibly don't open it. If such an entry point is available, I can start the heavy operation when the context menu is opened, and update menu labels/icons, activate or deactivate those items, or something.

And one more entry point, `chrome.contextMenus.onClosing` or something can cancel such a heavy operation if the context menu is closed before the operation finishes.