One older versions of konsole the tab section had a blank bit (right hand side) and if you dble clicked here you got a new tab. In 17.04.3 this bit of the tab bar is absent and the tabs occupy all the space and this dble-click gets new tab appears to have gone. I found this feature really useful and would like to have it back if possible. [please don't rule out the possibility I've missed something]
Qt5.9+ changed how they handle the tabbar - there's a new option in 17.12 Configure Konsole->TabBar->Expand Indiviual Tab Widths to FUll Window *** This bug has been marked as a duplicate of bug 380879 ***