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The code above does actually create a TabSheet with a few tabs (created with Vaadin Designer), and makes the first tab closeable. And know what? The browser internal to Eclipse allows to close the tab using the middle mouse button. Google Chrome allows using to close the tab using the middle mouse button.
If the component representing your tab is a Panel, you can add your MouseEvents.ClickListener there, so you could react to middle mouse button clicks. But it will not get events for the tab above the actual panel itself.