Starting from Qlik Sense February 2019 the Save button has been removed from the script editor.
The script editor now saves after every keystroke, so, saving manually is, generally, not necessary.
Latest changes can be reverted by pressing the Undo button:

It can be useful to do this in specific cases, for example to verify that the browser has not lost the connection with the Sense server.
Environment:
Resolution:
CTRL+S shortcut still forces a manual save in the Sense server script.