_NET_WM_OPAQUE_REGION
The EWMH property by which a window with an alpha channel declares which rectangles are actually solid, so the compositor can skip drawing everything behind them.
A window with an ARGB visual may be transparent anywhere, and the compositor has no idea where. It must assume the worst and draw the whole area behind it — every window underneath, the desktop, all of it — before blending the window on top. That did not matter while transparent windows were a curiosity, but with GTK 3's client-side decorations every ordinary application window acquired an alpha channel for its rounded corners and its shadow, and suddenly no window was solid. EWMH 1.5 answered with _NET_WM_OPAQUE_REGION: a list of rectangles, four numbers each, that the client sets on its top-level window and that promise the pixels inside are fully opaque.
The compositor can then strike those areas from its drawing list and treat the window like any solid one, with only a thin translucent rim left over. GTK 3 sets the property automatically, and Mutter and KWin read it. Wayland took the lesson with it and put the equivalent, wl_surface.set_opaque_region, in the core protocol from the start rather than as an afterthought.