function _activatePane(paneId) {
        if (paneId) {
            MainViewManager.setActivePaneId(paneId);
        } else {
            MainViewManager.focusActivePane();
        }
        // If fullPath corresonds to the current doc being viewed then opening the file won't
        // trigger a currentFileChange event, so we need to trigger a documentSelectionFocusChange
        // in this case to signify the selection focus has changed even though the current document has not.
        exports.trigger("documentSelectionFocusChange");
    }
@private
@param {string=} paneId - the Pane to activate