During some tests I accidentally un-tabbed a document (a new one without name).
I tried to tab it again, but this is not possible. The blue line in the tab area does not appear when moving the free floating document windows over this area.
Save it with a name did not help
Close it, and open it again integrated it into the tabs
IMHO this is a bug newly introduced (it does not appear in FM-15, 16) FRMAKER-12519