public class MyTool extends PanningSelectionTool {

	// ...

	@Override
	protected boolean handleDoubleClick(int button) {
		EditPart editPart = getCurrentViewer().findObjectAtExcluding(
				getLocation(),
				getExclusionSet(),
				getTargetingConditional());
		if(editPart instanceof ScalableFreeformRootEditPart) {
			// It's the RootEditPart, which usually has a ViewPort with Layers as its Figure.
			// This means the whitespace has received a double-click,
			// unless the setup is vastly different.
		}
		else {
			// Some other EditPart's Figure has been double-clicked on.
		}
		return super.handleDoubleClick(button);
	}
}