With long discussions on the JupyterLab side, it is becoming clear that the side panel and related features will be implemented in JLab console window. That is to say, the console window

1. will be destination of line by line execution (the runInConsole feature)
2. will be used to display transient_display_data (the transient_message plugin)

It is hard to say which interface is better but the console window has the advantage of saving all histories, which will fix #143 because the output of the preview magic will not overwrite the error message from previous statement.