This session property that begins as designer when Designer is first opened changes to browser at some point.
It may have to do with connection loss and reconnect between Designer and Gateway. I noticed this when scripting startup actions to hide components EXCEPT in Designer where we want to see them for configuration. When this session property changes incorrectly to browser, closing Designer and reopening gets us back to correct designer property.
Interestingly the gateway Perspective sessions page also switches these Designer sessions to display as chrome browser sessions when this happens. Two of these are actually Designer sessions: