function contextMenuOpenHandler() {
        var file = MainViewManager.getCurrentlyViewedFile(MainViewManager.ACTIVE_PANE);

        if (file) {
            var targetIndex  = MainViewManager.findInWorkingSet(MainViewManager.ACTIVE_PANE, file.fullPath),
                workingSetListSize = MainViewManager.getWorkingSetSize(MainViewManager.ACTIVE_PANE);

            if (targetIndex === workingSetListSize - 1) { // hide "Close Others Below" if the last file in Working Files is selected
                CommandManager.get(closeBelow).setEnabled(false);
            } else {
                CommandManager.get(closeBelow).setEnabled(true);
            }

            if (workingSetListSize === 1) { // hide "Close Others" if there is only one file in Working Files
                CommandManager.get(closeOthers).setEnabled(false);
            } else {
                CommandManager.get(closeOthers).setEnabled(true);
            }

            if (targetIndex === 0) { // hide "Close Others Above" if the first file in Working Files is selected
                CommandManager.get(closeAbove).setEnabled(false);
            } else {
                CommandManager.get(closeAbove).setEnabled(true);
            }
        }
    }
Enable/Disable the menu items depending on which document is selected in the working set