Login to Sakai.
Click on the maximise button on any tool.
You should see the full screen message at the top of the screen (see screenshot).
Click on the message. You should be returned to the full portal view.
Full screen message doesn't make it clear how to get out of full screen mode. See SAK-44013, particularly Adam Hauerwas's comment. Just removing the max button and making the text read as he suggests seems the clearer approach.