Terminal tabs undraggable

At some point, the tabs on the terminal screen have become undraggable.  I recently reinstalled on a new laptop and it's happened there as well.  I can't see any obvious settings that affect this.  The only settings I've changed are:

* more than 10 tabs

* disable tabs in one row

* enable new tabs to the right

I've disabled each and every one of these and none of them cause the behaviour to revert back.  I did notice that the change happened after one of these settings was changed, however, but not which one, but the fact that changing them all back does nothing is itself suspicious that that may not be the issue. 

Any ideas?  It's driving me nuts not being to organise my tabs...