This was a kludgey hack that never managed to land upstream, yet utterly dominated (for a brief moment) the headspace of the early linux desktop.
It's funny how quickly things were moving at the time. In the mid 90's, GUI design elements were still in their infancy. Even basic stuff like "what do windows do?" was in flux. Traditional X window managers hadn't settled on anything like a regular usage model: twm was still in regular use, fvwm mostly cloned its UI, Sun was still defaulting to OpenWindows which was pretty and clever but sort of an evolutionary dead end, and other commercial unixes were running Motif which was a lot like a monochrome Windows 3.1 that used too many pixels. Macs were still stuck in the only-one-foreground-app-is-enough model with System 7 and had nothing to offer.
Then Windows 95 landed like a bomb: there was a CLOSE button in the corner of the window finally! And there was a start menu and a little status bar! And that's what we all decided we wanted, really badly. So it got cloned and picked up pervasively. Basically everyone not already part of one of the X11 camps was running this.
But the window was small. KDE kicked off mere months later, Gnome followed quickly after that, and we all forgot about fvwm95. But we for sure all remember it.