Haiku made them magnetic. In classic BeOS you had to manually position the windows on top of each other, then hold shift and manually drag the tab over from the left. And they wouldn't be tethered together: moving one window wouldn't move the other. It was annoying enough that I never bothered. By the end of BeOS's life I was mostly using the Mac OS 8 titlebar style (accessed by holding down all the modifier keys and pressing the Be menu).