This happens when IO is reused:
1) output does not contain any line from previous operation
2) out, err and in are closed
3) tab name is still presented in bold as if it would be still active

Seems that problem is in this part of OutWriter:
    public synchronized boolean isClosed() {
        if (checkError() || storage == null || storage.isClosed()) {
            return true;
        } else {
            return closed;
        }
    }

storage.isClosed() probably return false only if there is some output written - consequence of this is no closed event
emitted if there were no lines printed to the pane.