[client] x11: set _NET_WM_PID

Fixes #1013

client/displayservers/X11/x11.c

@@ -350,6 +350,21 @@ static bool x11Init(const LG_DSInitParams params)
   // check for Extended Window Manager Hints support
   x11CheckEWMHSupport();
 
+  if (x11atoms._NET_WM_PID)
+  {
+    pid_t pid = getpid();
+    XChangeProperty(
+      x11.display,
+      x11.window,
+      x11atoms._NET_WM_PID,
+      XA_CARDINAL,
+      32,
+      PropModeReplace,
+      (unsigned char *)&pid,
+      1
+    );
+  }
+
   if (params.borderless)
   {
     if (x11atoms._MOTIF_WM_HINTS)