Now this opens as an undocked panel upon start.
Come on people! Just remove that, please.
I suspect that you have used Jessie’s workaround to get rid of proliferating notifications and left the last one as an undocked window before you closed Rhino. Easy to fix, but save your work first (more on that later).
Just delete the undocked window, then go to the panel settings and re-tick notifications to get them back into the panel. Once they show up, close Rhino and reopen to ensure they stick in the right place.
If on SR17 or earlier don’t do the other thing I tried: dragging the undocked notifications back into the panel. Totally screwed the UI, leaving an “open file” command running that would not respond, totally black viewport and no way to exit other than cancelling from Task Manager. On restart Rhino was repeatably still unusable and required a Repair. However, on a dev build of SR18 (which may or may not include the notifications fix mentioned by Wim) dragging the notifications back into the panel was fine.
Oh, and in the SR18 dev build you can turn notifications off in the panel by unticking them. Didn’t used to be able to do that.