Due to a crashing bug in Qt, we have disabled dragging and dropping of tabs unless the user specifically enables that via an environment variable (crashes are rare but do happen deep inside Qt from trying to reorder tabs too quickly or dragging them outside of the tabbar).
See:
https://bugreports.qt.io/browse/QTBUG-74478
The environment variable is called:
SIGIL_ALLOW_TAB_MOVEMENT
Setting this to 1 or true or any value really, will reenable tab dragging and dropping. If you do a lot of that be careful to not drag things too quickly or too far.