_NET_SUPPORTING_WM_CHECK

The root window outlives the window manager. Everything it wrote there — _NET_SUPPORTED, _NET_CLIENT_LIST, _NET_CURRENT_DESKTOP — remains after a crash and looks as valid as ever. EWMH's answer is a witness: the window manager creates a small, never-mapped child of the root and sets _NET_SUPPORTING_WM_CHECK on both the root and the child, each holding the child's ID. A program that wants to know the state reads the property on the root, follows the ID, and checks that the child exists and points back at itself. If the window manager dies, its windows vanish with its connection, the chain breaks, and the stale root value is exposed as garbage.

The child window also carries _NET_WM_NAME with the window manager's name, which is how wmctrl -m and assorted scripts know whether they are talking to Openbox, KWin or Mutter. Next to it on the root sits _NET_SUPPORTED: the list of every _NET_ atom the window manager understands, so a program can ask before requesting fullscreen or a strut rather than send and hope. GTK and Qt run the check at startup and cache the answer. The pattern came from GNOME's _WIN_SUPPORTING_WM_CHECK in 1998, and it beats the ICCCM-orthodox WM_Sn selection on exactly one point: it says what the window manager can do, not merely that it exists.