Comment 1 for bug 1373773

The fact that the webapp container exposes a default contextual menu is intended. It’s unfortunate that it doesn’t expose a command-line option to optionally disable it though.

You have a couple of options there:
 1) Ensure your web application doesn’t emit "contextmenu" DOM events (this is what the selection mechanism uses), i.e. if it swallows them they will never reach the container
 2) Instead of using the webapp container, build a simple QML application that embeds a WebView (import it from Ubuntu.Web), which by default has the selection mechanism disabled

I’ll leave this bug open so we can consider adding a command-line option to the webapp-container app to optionally disable the selection.