Alt-drag
The X window-manager habit of moving a window by holding Alt and dragging anywhere inside it, instead of hunting for the title bar.
The mechanics are simple. The window manager takes a passive grab on Mod1 — Alt on a PC — plus the left mouse button, so the click never reaches the application, then moves the window with each pointer motion. With the right or middle button it resizes instead, from whichever corner is nearest. Openbox, Fluxbox, xfwm4, KWin, dwm and i3 have all done it by default, and TinyWM consists of little else.
The cost is that Alt-click becomes unavailable to applications that want it, and paint programs, 3D tools and emulators complained. GNOME moved the binding to the Super key, and KDE Plasma followed in version 6. Neither Windows nor macOS ever built it in, so utilities such as AltDrag filled the gap for people who had got used to it and could not get unused to it.