An update. Helpful folks at #fluxbox identified what it was. The option to start tabbing was by default assigned to a different key/mouse combination.
The line in ~/.fluxbox/keys was responsible:
Code:
OnTitlebar Control Mouse1 :StartTabbing
If one wants to change it to use the middle mouse button, it needs to be changed to:
Code:
OnTitlebar Mouse2 :startTabbing
and change another setting that uses Mouse2
Code:
OnTitlebar Mouse2 :Lower
Problem solved - thanks to the guys #fluxbox