Always on top
The window setting that keeps one window above all the others regardless of focus: a small feature that exposes how the window manager actually orders its stack.
In a stacking window manager, windows sit in an order where the most recently clicked one ends up on top. "Always on top" breaks that rule by splitting the stack into layers: normal windows in the middle, pinned windows in a layer above, and often a layer below for things like desktop icons. Within each layer the usual ordering applies, so two pinned windows can still hide each other. fvwm made the layers explicit with its Layer command, where 4 is normal and 6 is on top; most others settled for a checkbox in the window menu called "Keep above others" or something similar.
EWMH standardised the state around 2000 as _NET_WM_STATE_ABOVE, with _BELOW as its opposite, and noted explicitly that it is meant as a user preference, not something applications should request for themselves. Windows has had it since 3.1 as HWND_TOPMOST but never exposed it in the window menu, which spawned a whole genre of small utilities from DeskPins to PowerToys' "Always On Top" in 2021. Tiling window managers barely have the problem: where windows do not overlap there is no "top", until something floats.