How can the context menu in WKWebView on the Mac be modified or overridden?

You can intercept context menu items of the WKWebView class by subclassing it and implementing the willOpenMenu method like this:

class MyWebView: WKWebView {

    override func willOpenMenu(_ menu: NSMenu, with event: NSEvent) {
        for menuItem in menu.items {
            if  menuItem.identifier?.rawValue == "WKMenuItemIdentifierDownloadImage" ||
                menuItem.identifier?.rawValue == "WKMenuItemIdentifierDownloadLinkedFile" {
                menuItem.action = #selector(menuClick(_:))
                menuItem.target = self
            }
        }
    }

    @objc func menuClick(_ sender: AnyObject) {
        if let menuItem = sender as? NSMenuItem {
            Swift.print("Menu \(menuItem.title) clicked")
        }
    }
}

Instead of this you can also simply hide the menu items with menuItem.isHidden = true

Detecting the chosen menu item is one thing, but knowing what the user actually clicked in the WKWebView control is the next challenge :)

It's also possible to add new menu items to the menu.items array.

You can do this by intercepting the contextmenu event in your javascript, reporting the event back to your OSX container through a scriptMessageHandler, then popping up a menu from OSX. You can pass context back through the body field of the script message to show an appropriate menu, or use a different handler for each one.

Setting up callback handler in Objective C:

WKUserContentController *contentController = [[WKUserContentController alloc]init];
[contentController addScriptMessageHandler:self name:@"callbackHandler"];
config.userContentController = contentController;
self.mainWebView = [[WKWebView alloc] initWithFrame:self.view.frame configuration:config];

Javascript code using jquery:

$(nodeId).on("contextmenu", function (evt) {
   window.webkit.messageHandlers.callbackHandler.postMessage({body: "..."});
   evt.preventDefault();
});

Responding to it from Objective C:

-(void)userContentController:(WKUserContentController *)userContentController didReceiveScriptMessage:(WKScriptMessage *)message
{
    if ([message.name isEqualToString:@"callbackHandler"]) {
      [self popupMenu:message.body];
    }
}

-(void)popupMenu:(NSString *)context {
    NSMenu *theMenu = [[NSMenu alloc] initWithTitle:@"Context Menu"];
    [theMenu insertItemWithTitle:@"Beep" action:@selector(beep:) keyEquivalent:@"" atIndex:0];
    [theMenu insertItemWithTitle:@"Honk" action:@selector(honk:) keyEquivalent:@"" atIndex:1];
    [theMenu popUpMenuPositioningItem:theMenu.itemArray[0] atLocation:NSPointFromCGPoint(CGPointMake(0,0)) inView:self.view];
}

-(void)beep:(id)val {
    NSLog(@"got beep %@", val);
}

-(void)honk:(id)val {
    NSLog(@"got honk %@", val);
}